Thanks for letting us know about this. It's actually not from the banner ads -- it happens when a user uploads a team logo or banner from some websites that are known to distribute malware.
It's not a particularly dangerous situation for our users. However, as the warning message can scare people off, we remove the offending logo or banner and modify our code so that no more logos or banners can be uploaded from that site.
This particular instance should now be resolved. Please let me know via BB-Mail if you continue to see this warning. Thanks!