Well, don't they say prevention is better than cure? The solution of just deleting spam posts works only insofar as the spam appears on the forum, then there is a delay between the reporting of it, then you receiving the report and taking the action - by that time some damage has already been done.
If you look at the whoisonline list you will see several dozen posters with names like hhkj, ggbts modifying their ignore list or modifying their posts. These are spampots and spiders. One way to deal with them is: Ban all their entries and then, introduce the following into the registration procedure
(a) E-mail validation for all new users
(b) Human user validation for all new users
This will take only seconds for you to implement, and voila the problem will be gone forever.