A little belatedly, as cassa says, you are within your rights to trim any overhang, back to the property boundary. Prior to doing so, it would be advisable to write to your neighbour, telling him/her of your intention to do so.
This would give them the opportunity to do it before you, and then if you get no response, go ahead and cut them back.
Send your letter of intention by recorded delivery.
Any clippings remain the property of your neighbour, and should be offered to them.
If they refuse to accept same, dispose of accordingly.