It's nowhere near as bad as it used to be - during the first half of 2010, we regularly had more new spam posts than actual posts by a factor of at least three. These days, I'm lucky if I get to delete one spam thread a week.
If anyone is interested, the current stats are: 5059 deleted posts and 1619 deleted threads on the forum. Now these are "soft" deletes, which means that they can be retrieved. I have no way of finding out how many posts and threads have been deleted in total (soft and hard deletes), but it's bound to be a lot more (particularly as this is post #697,061).
I suppose it's possible to go back through every post and see which I can view with my permissions, and which I can't, but I don't want to do it manually, and I can't be bothered writing a script.
As an aside, this is the earliest accessible post: http://zelaron.com/forum/showpost.php?p=4