Hi halsafar,
I am dealing with these exact problems these days.
The case was the next one:
Forum with around 4000 user account with around 85% of them bots.
I needed to,
1 - Forbid new bots to come in.
2 - delete old spam messages.
I took the next steps.
delete 0post users - http://punbb.informer.com/forums/post/143634/#p143634
delete every bot user and it´s posts - I didn´t find a 1-click way. I am doing it in a custom way.
I ordered users by posts and IP address. In my case it happened that most of the spam comments come from the same IP addresses so it was not so difficult to get rid of most of it.
There are some strategies that could help you, for example to know the approximate date your forum was spotted by spam bots, so you can search for user created after that date. Or to look for some words.
As I am pretty new to this theme I would like to here more opinions and suggestions.
Scripts don´t look like very difficult so if someone explains me some patterns to look for, I could try to program something.
Saludos and thaks to all