What is that criteria? You won't be allowed to earn any Coliseum normal kills or Ulduar 25 hard-mode kills. You are still allowed to PuG those fights, though the limit is 10 players over time, or five in the same fight (in other words, if 10 players of your guild accomplish a kill in 25-man Coliseum, or five of them do it in the same raid, you're off the 10-man strict list). That's designed to make sure that the list stays as clean as possible. You'll have to watch recruits, too -- anyone who enters your guild with a certain achievement has it counted towards that total of 10 players as well. All the other info you need to know about the listings are over in their FAQ.
Players may find other ways around the limit (there is, of course, higher level gear now available from Emblems that 10-player guilds are still able to get), but for now, that's where GuildOx is putting the limit, so if you want to stay on the list, follow their rules. Good luck to all of the strictly 10-man guilds out there.