Recently there have been an influx of nuisance obscene adverts in my message board. The ips changed constantly.

The ips of latest two are as follows:
1) 2009/10/01 11:11:23 217.6.66.147
1 Oct, 11:11:27 Germany mail.deilmann.de 1 Windows XP Explorer 6.0
2) 2009/10/01 11:11:22 85.99.99.19
1 Oct, 11:11:26 Turkey gateway.cankaya.edu.tr 1 Windows XP Explorer 6.0

Would appreciate if someone could advise on how to get rid of influx of nuisance adverts.

Thank you.