I started a "spam here" thread in jest, but Fred made a very good point in it:
Spam targets are dropped if they don't return clicks. Remember that when you are tempted to click on one of the links. I know, nobody wants to admit they click on spam, but if it wasn't happening, the spam wouldn't be increasing.
To an extent spam is simply on the rise everywhere, other forums are experiencing the same, but Fred does make a good point in that, even though you safely run Linux, do not follow these links. Believe me you are not missing anything, and if you just want to see the **** then visit the sites on your own volition, why pay some spammer?
I have a thought, along with not allowing guests to post. One forum I frequent, a vbulletin powered forum, has an option to not allow a new user to post any links until a set number of posts have been reached. This makes it harder on the newer user, but you could set the number at 10 and it really shouldnt cause too many hardships while stopping much of this drive by spamming. This has really worked for the forum in question. I do not know if phpBB provides this option though.