Soldiers blocked from applying to special forces by commanding officers:

That's happened in the US for years. The argument about drawing away talent and resources goes back to at least the Peninsular War with Wellington. I'd imagine it pre-dates The Rifles by a fortnight or a thousand.

Give it five years and you'll see the same story but with new quotes and personas.
The big reasons this was happening (not saying I agree with them) was because there are units that are in high demand for troops good or bad, for example combat engineers. Another reason was because guys were going on advanced courses such as MFP, Advanced Reconnaissance, Patrol Pathfinder etc and than putting in applications to the command. It takes a lot of time and money for people to get qualified on these courses and the COs just wanted to get something out of the guys before they left.
There was a guy from my old commo unit who was supposed to leave the week of Thanksgiving for selection. The BN CSM called up a CSM at Bragg, and spun a sob story about deploying soon at only 65% strength, which was a half truth (we were 9 weeks out, but at 95% strength). The SF CSM believed the bullshit, and that specialist's trip to selection was cancelled 48hrs before his flight.

This was at a time (2004-2005) when sending men to selection/SFQC took priority even over fencing and deployment dates. The lesson here is that lesser men in Mother Army will always go to great lengths to keep the best troops for themselves and not let those troops strive for greatness.
This has always happened up here, especially once the command was stood up and manning of all the new units began. Career Managers have gotten "involved" as well. Things will never change.
I believe the selection that can't be blocked is for JSOC units.

In Poland you can go for selection even during your holidays(or even sick leave), but even If you pass the selection, you have to ask Land Forces/Navy/Air Force commander to let you go to Special Forces branch. And he can say no. I read an article about that some time ago(year or two), and Land Forces commander approved only requests from NCO's who passed selection and wanted to move to Special Forces branch, while he did not approve officers requests. No idea however what is the rate of refused requests. So, even If you pass selection you can stuck in your unit because higher command will not let you go to SF.
Mack that piece is no longer active. Since Poland is on its route to becoming a "SOF Framework Nation", resuplying it's units with man power has become a priority. So now if a candidate has a "green light" to go eighter to GROM, Formoza or the Regiment - his commander MUST give him a GTG. Of course there is always the Murphys law of "your paperwork got lost" or "we're still awaiting your reassignment orders"(even though you got the call and they said they were sent 3 months ago), or he can simply put you in a shitty job that unables you to trasition for the next 18-months. Then it's still an administrive suck fest, because they say the spot is no longer vacant and you need to wait 'till a new position opens up.

Right now DWS is in the limelight, so MON needs to make'em look as good as it can get. You know the rest of this sad story.
That is why our SOF has granted access for guys in the NSR corps - they are not obligated to go throught all that shit stuff, but the paperwork takes longer.
So for our guys it's kinda like Fleckers Pilgrims, but not through doubt but the administritive heartlessness ;-)
So now if a candidate has a "green light" to go eighter to GROM, Formoza or the Regiment - his commander MUST give him a GTG.

His commander maybe, but branch commander? He can do whatever he want unless chief of general staff or MOD will tell him to do otherwise. But I was talking only about situation from the past, so maybe now branch commanders always give green light. So, you may be right.
As stated earlier, filling the SOF ranks has now taken priority - due to the NATO thingy promisse and, as always, looking like we know what to do.
Of course there is no guarante that things will go back to the great old ways when it all dies down. And there is a good chance it wil happen, since the Prez decided we're no longer putting our eggs in the expeditionary basket. So if someone wants to get into POLSOF, he or she had better do it before the end of 2014.
Applying for selection to a JSOC unit is pretty much automatic from what I remember. You reach out to the JSOC recruiters and they get the ball rolling. Your command can eat shit. But there are personnel who will do what they can fuck you over and keep you there.

Ranger and SF is a whole 'nother case. Pretty sure it has to be approved, right?
When I was in batt., we didn't have anyone in good standing go to SFAS, but we did have a fair amount go take the long walk. From what I saw, the amount of pushback received depended entirely on how many guys were going at any given time. One or two guys going? No big deal. Nine NCO's from the same company going? Major pushback. Things like "You better get selected, because if you don't you wont have a job when you come back" were said.