Get them a stanley knife and when they ask what its for you can carve "This is for supporting such a soul-less football club you ****" into their back and/or face.
Cheap and effective and they can always use it for DIY jobs when they've rinsed the blood off.
They are very very touchy if you ask them who they supported before the money turned up. May have asked this in a pub on the Kings Road last week. Touchy!
I love that inflatable European Cup though. Only problem is the twat might actually like it and not get the joke.