Every forum admin is used to seeing supposed offers for cheap ipods and mobile phones on their forums, supposedly from trading companies.
After some research, Security Watch has found that a lot of this spam originates from just two IP addresses:
Checking up the IP history of one forum, no less than 20 banned users had used these two IP addresses alone to post ipod and nokia mobile phone spam.
Of course, these messages are not simply spam - they are also scams.
Using the spam to entice users to contact the Nigerians on free email addresses, they will then encourage payment for the goods - which will never materialise.
All the more recent to ban these two IP addresses from your forum.