If you try to cover the smell, the cat will just think that you're another cat, covering up his possession markings. When toms spray, that's what they're doing (flashback to Red Dwarf where the Cat [humanoid evolved from cats] is going round the ship with an aerosol spray, going "sshhpprrrrp, that's mine... and that. sshhppprrrrrp, all that, except that bit"). Kitty is telling you that although you use it, the case is really his, he just lets you borrow it. And in case you should meet any other cats whilst out with it, he's left his calling card so that any other cat will know you belong to him.

Kitties don't really like citrus smells so try a little orange oil or lemon oil along the edge of the case, or leave some peel in there for a while. That should prevent further 'attacks' once you have de-ponged your case. I would recommend the removal and replacement method. Although there are smell removal products that are very very good, sometimes they just aren't enough.

LTS