Yeah playoff roster management is screwed up in the game. I don't understand why Markus has not fixed it. The last straw for me (solo league) was a few seasons ago Oakland picked up a player from Milwaukee on waivers during the playoffs and he went on to get the winning hit in the WS. Even worse Milwaukee made the playoffs and this player played for them in the first round. It really spoiled the season for me.
I can't stop call ups but I do stop waivers from working by setting them to 31 days (DFA too) during the playoffs. At least players don't switch teams.