Hi ,
when I voted down a spam question, my reputation was decreased by one. I think after a question has been deleted due to spam, the reputation which was removed should be added back ;)
what do you think ?
Your reputation will be restored next time your rep is recalculated. If you want to force this, you can go to https://stackoverflow.com/reputation and click the button at the bottom.
Why don't you just flag it as spam? No need to downvote.
But I agree, when it is removed, your rep ought to be increased again.