You cannot touch them as they do not belong to you. You need to get the police involved.
If they are on your property I would be moving them. Surely you have the right to do so after asking them and giving them enough time to act.
Seen as they dont want to do so, to me that means they dont want them.
Put up a sign saying free you take away. Or just throw them back over the fence one at a time. To me its the same if your neighbours tree hangs over onto your side of the fence you can trim it and throw the branchs back onto their side as it is their tree.
Regards