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.
Exactly. Bitcoin can only function correctly should the majority of hashing power remain honest. In this context, miners following such regulations are dishonest because they do not always build on the longest chain.