Providing you are at the end of any contract term, you can downgrade to PAYG, or cancell completely.
Then if you pay by direct debit simply instruct your bank to cancel your direct debit agreemant with them.
If you pay by credit card write to Tiscali & instruct them not to take any more payments from your card.
If they then continue to take any money contact your credit card company with a copy of the letter & they will do the rest.
When I cancelled my account I found that the letter did the trick.