The OP got my blood pressure up, which as most of you know is not something I really desire right now.
As Mike and others said, it really shouldn't make any difference whether it's May or September; if someone deserve to get jacked, jack 'em. Yes, I might be a little more lenient in a playoff game, but NOT AT THE RISK OF THE INTEGRITY OF THE GAME.
I would assume you had a plate conference before this game started. Someone would be representing the team as a coach or captain or something. Talk to them (and AS STATED, HAVE THEM come to YOU) in a calm, professional manner.
I think many of us have seen the consequences of too few ejections and lack of assertiveness by some umpires. It can come back to bite you, sometimes in the same game, sometimes months or years later (Dear God, why did you give players such an excellent slective memory?). That doesn't men we have to be trigger happy - but playoffs and having only 9 players be damned. If the situation warrants an ejection, then jack 'em.