I've been trying to remove the receiver extensions on a couple of Spikes lowers.  The lock nuts are staked in place so I used a Dremel (yes, I know all of the Dremel tool jokes) to grind out the staking dimples, but I still can't budge the lock nuts.  I've broken the nubs off of two different AR tools trying.  I have the receiver in a mag well type vise block so there's a lot of unsupported space between the anchor point and where I'm applying the force - I'm afraid of distorting the receiver.  And I'm hesitant about using heat for the same reason.
Any suggestions other than cutting the nut off, and most likely having to replace both the nut and the extension tube?

Heat them up.  They probably used loctite.

Why use a dremel?  Just take a punch and push the metal back over.  

That said, sounds like it might be loc-tited as well so you'll want to heat it like mentioned above.

If it is a lower assembled by Spike's, it will not have Loctite on it.  Spike's torques their extension nuts and uses staking to secure it.  

I support the lower with a lower receiver vise block in the mag and use my body against the extension tube while I remove the nut to keep everything from flexing.  Just make sure you are using a good RE nut tool and have the teeth firmly planted in the nut.