the contacts on the relay in the fuse box may be sticking together( due to the high current arcing across the contacts and efectivly welding them together). identify the relay and bring the car up to temp and let the fan kick in. then pull out the relayand the fan should stop. (depending weither is connected to normaly open or normaly closed contacts.) prise open relay cover and check if contact a stuck. if the are care fully prise them open and clean contacts with a needle file or an emery board, but be carefull when dispantling as the is a spring connected to the contact/coil mechanism and can easliy be lost.

failing that pop along to for and just buy a new relay probably cost less than a tenner