Players announce they are going to check it down?

[mouseman]

Yesterday in a MTT I had 575 chips. The blinds were 250/500. I got jacks in MP and went all in. CO and button called as did SB and BB. They decided amongst each other out loud that they were going to check it down. This happened about 2 hands earlier as well in a hand that I wasn't involved in and I told them that this shouldn't be done. This is my normal home game and the 4 players that were checking it down I have never seen at this particular game before. I'm pretty sure that this shouldn't be done but is it illegal. If this happens again is there any way to enforce this rule like making them muck their hands?