If you are one of the housemates, why did you apply for internet without asking everyone to chip in for the monthly payment? That way, those paying ones can use it as they wish.

If you are the sole payee for the internet, then change password and don't share. Tell them you need the bandwith for your use. Or turn security on when you are at home and off when you leave so that they can use.

You should be selfish at times. In fact, secretly blocking or limiting is already selfish. Why not try plain honesty and be upfront with them? You think they won't notice when their download speed suddenly drop so much? Making them suspect you will be even worse.