Unless the owner/miner of the coins agrees for those to be withhold and the operator pays an interest for holding them, then blocking payments is in fact stealing.
That without mentioning other damages to the miner like in the case bitcoin goes down in price and the miner has to pay electricity and other bills in USD or any other currency.
The longer the operator keeps the coins for himself, the more trouble he will have.
So yes, blocking payments without the miner consent is in fact stealing.