My discussion forum which was becoming more and more popular by the day has been hacked into.

I think some of the spammers that I banned got upset and did this.

I got complaints from new members unable to register or rather they'd get an email confirming their registration and when they'd click on the link they're taken/redirected to a pronographic website. I checked with a disposable yahoo email account and to my horror it turned out to be true.

It's a Simple Machine Forum I got for free.

What to do and... anyone with any advice?

Say if I were to go for a paid one can this problem reoccur?

Thanks

You dont give enough data..

How was the site hacked ??

Was it simply compromised admin passwords or has the forum code been hacked ??

What do the forum / server logs say ??

Is the SMF code fully patched and up to date ??

Was it on a dedicated box, or shared hosting ??

For all we know you could have an idiot password for the admin that someone guessed.. In that case its not the forums fault ? Conversely they could have found some scripting weakness thats not known to the general public and that wont be easy to fix.. Or again they could have total access to your hosting..

Too many variables from the limited information you have presented.