there are a number of ways that the tenancy contract could be brought to an end in this situ.
1) if tenant is damaging property
2) if tenant is in arrears
3) if tenant is carrying out illegal activities
4) if tenant abandons property
1) and 3) can normally be solved quickly in court – seek legal advice pronto.
For 2) to be effective they have to be 2 month in arrears before application can be made to court, but then its pretty quick.
If its number 4 then its just a case of being sure they have abandoned, ie, no rent paid, stuff moved out, take fotos and be really sure they have abandoned then just change the locks fix up and re -rent.