Was it an initial standard AST with a 6 month period then letting on a month to month basis? If so are they within or outside the 6 months?
Make sure everything is done legally and properly so there is no case for unlawful eviction. Sometimes it might work to ask them to leave with a reasonable notice period and then try to claim rent back but if that doesn't work and in general the best way is the legal way to avoid any further problems.
This may help depending on the circumstances...
http://www.letlink.co.uk/letting-factsheets/fa ctsheets/factsheet-8---claims-for-possession-t he-section-8-notice.html
http://www.clickdocs.co.uk/section-8-notice.ht m
A Citizen's Advice Bureau may be able to provide more specific advice if you take the tenancy agreement down to see them.
Please note that even tenants on informal tenancies (ie no written agreement) have some rights under law.