Student spilt pre occupation

Other

A fixed term joint tenancy is set up including holding deposits paid, for 6 students sharing with occupation starting in June 2014.
Now the group has fallen out and split 3/3. Neither 3 agree to give up the house and are not talking to each other. At least one person is now known to be taking drugs and I would prefer they did not occupy the house. As occupation has not yet started, can the Landlord do anything legally to either stop the whole 6 students occupying (IE stop the tenancy) or replace just three of them for the sake of harmony???