The obvious first step is a poilte word - this usually solves the problem - give it a couple of tries and remain friendly if possible, no-one wants grief with neighbours if it can be avoided. If that doesn't work, post a note advising, in as friendly terms as possible, that the fence does belong to you, and you are now making your request official. The reason for posting is that you use Registered Mail, as proof that they have received your request. If it continues, it's time to ask your solicitor to fire off a warning letter, and hopefully that will work. If not, he will advise your rights on the issue, but try to exhaust all reasonable means of resolution, and go for litigation only when everything else has failed. Anyone continuing to damage your property after a polite, and then firm request to stop, may be beyond being reasonable, so you've nothing to lose by taking the legal path from there.