Delete Spam & advertising message threads instead of just closing them
I think the reason why spammers spam here is because the moderators only close the threads, which still allows people to click on their links and allow them free advertising. If you delete the threads, you deny them the attention they are after, and therefore they would find it more of a time & money loss factor for them to post it here.
Well I always remove them rather than close them. I agree that closing them rather than deleting them doesn't solve anything. I'm not sure where you are witnessing this but I know in the forums I moderate I always remove them.