I'd need to know what exactly they're doing in order to block it. You should know that several servers have been getting attacked the last few days by someone spamming A2S_INFO queries from spoofed IP addresses.

If that's what happening, the best thing you could do right now is rate limit queries using an IP tables rule. The downside is while an attack is occuring, people won't be able to see the server, but it won't lag anyone playing either.