I've got a website running with a forum attached. Problem is, I already have spam coming in at least a few times a week.

The forum software I'm running is phpBB2. I can either choose that one, or go with SMF.

Since the site has quite literally just begun, I won't be losing any important data if I had to change over.

Anyone have any suggestions about which one is better against spam?

Thanks

It depends on how your forum is being spammed.

if it's real people then there's not much you cam do about. I seem to remember phpBB2 records data like IP, email address, maybe has email verification.
Ban the user
inform the email ISP that the user is hiding behind their network
determine the ISP the user is connecting from, using the remote address - inform that ISP and complain about an abuser.
[I'm assuming posters have to be first registered and signed in. If not - implement this ASAP, not doing so would be advertising yourself to spammers]

I've never had a problem with spam on forums. I see very little spam on this forum. I make sure I collect as much data as possible from users.

Moderation is fairly simple in phpBB2, so deleting the spam shouldn't be too bad.

If it is automated spam, read on:

phpBB2 is fairly commonplace, so more spam bots will have been developed for phpBB2 than most other forums.

It's not difficult to make a spam bot, most of it just sending form data.
even validating sign ups and posts by using image verification is getting more sophisticated as OCR software improves.

Uuhhh Stitch.. aren't U signed up with me over..er..uh.... On the dark side??
Wouldn't you simply go there and post this in reverse

"Hey Fisheads... which is the easiest Forum app to break.. PhpBB? Which do you guys find easier to break?"

Then you'd REALLY have the answer my young Jedi... tsk tsk.... U must learn to USE the force better Young Stitch