Ask them to add the site to their safe sites list - fixes the problem and they get to keep their firewall intact.

More often than not it's Norton Internet Security; you can disable the checks through your control panel, and some members say that if you're not experiencing spam through alternate referrals theres no point of having it on; myself, I don't like the idea of nerfing security on my forum because a user's security suite is going haywire with beefing security...

Thanks for the tip.