Is it technically possible for miners to "freeze" certain bitcoin addresses by rejecting all transactions to/from a list of addresses? For example if all (or 50%+) miners agree or are forced to by some new regulation to block any transaction from an address that is known to contain stolen funds. If this is possible, then it would open up for all types of regulations forcing miners to become gatekeepers and blocking "bad" transactions.
In short, yes, they can.
Will they?
Not unless they got a gun to their head or some government imposed it claiming "terrorist attacks"
Apart from that, AFAIK the 50%+ attack will not serve the attacker to anything else other than double spends and preventing others from transacting.
But then again if the word gets out that there is a 50+ on one entity/group, changes will happen very soon as to get things back to normal.