Clean uninstall HOWTO

[Codex]

You can't "de-activate" your copy of CoD, it's tied to your STEAM account for ever.

EDIT: You wont run out of activations for CoD however, you have a max of 3 actiavtions so you can install YOUR copy of CoD on three different PC's. Once you use those 3, you will get 1 additional activation / month up to a maximum of 3. You can install CoD on the same PC with no limits.

UBISoft FAQ for CoD:

Is it possible to revoke an installation?
Revocations are not considered, since users get one new activation per month, so if you do somehow manage to burn through your three activations, you only need to wait one month or contact support.