Micca's oil of cloves suggestion is a good one - if you don't mind being reminded of toothace every time you go near the tree.
Garden centres also sell a pepper-type spray which is meant to keep the moggies away from your veggies - that might be worth squirting on the tree.
Failing that, how about widening Loki's circle and doing away with that old cliche of a fairy on the top of the tree (ouch!)?
§;^)