removing one of 4 tenants

Possession - Section 8 and Section 21

Hi,

The house has 4 named tenants on the tenancy agreement, and they all contribute to their own joint bank account etc cover rent and their bills. It has only very recently come to our notice that one of the tenants has become of an issue, with some behaviours almost becoming harrasment. The tenancy was started on 1st Feb 2013 for a period of 6 months, a section 21 has not been served, and the intenttion had been to let the tenancy roll on. My question is am I able to serve notice on one tenant, or can I serve section 21 on all and sign a new agreement with the remaining three tenants? are there any risks on this.

The other worry I have, which I am not sure if it is my own error, is around the deposit, I went through all the right things on the deposit protection, but I have a memory that the deposit came from one of the tenants, and so this one tenant is noted on the deposit certificate, but I now think this tenant had given the money before hand to the previous tenant. At the beginning of the tenancy I had asked (only verbal) what was the split on the deposit, but advised it was coming from one named tenant. I think it may have been the case that this one tenant had lent the money to the other tenants, but possibly took back what should have been their original contribution, so not sure where I might stand on this one, as the oe tenant I want to serve notice on is probably going to become a problem.

Thanks in advance for any help.