Don't want tenants partner

Tenancy Types and Management

AST tenant since 2008 suing us as fell down stairs, moaning about neightbours (in my opinion unjustifiably), I was going to wait til Insurance sorted out claim and then evict with S21 however it has come to light that her partner has moved in probably at Christmas. She did not notify me until I mentioned it. He dropped a letter through the letterbox on Friday saying he is now a joint tenant giving name & DOB. I do not want to do another AST with both on it for 6 months, I am not happy about him living there as I know nothing about him, and don't particularly want her there after claim is settled one way or the other. They will "know their rights" so I have to be careful.

What do I do now???? Many thanks