If its a mink you can kill it, either by shooting or trapping or by cage trapping then shooting it. I remember i was asked to deal with badgers once and as they are protected i called what was then defra and was told the customer can apply for a tempory licence to interfear with them in order to alleaviate damage. There were three stages of licence, 1) put a trap door over their sets so that once out they cant go back in and hence they f**k off, 2) trap and release somewhere else, 3) shoot. So first of all establish for certain if its a mink or a otter. If its an otter get on the phone.