Simple really.
If a bard does it but doesn't bother anyone and/or is careful with other players, who cares really.
If a bard does it and someone has a problem with it, ask the bard to stop. This should be the first option for anyone having issues with it to limit GM/guide work: talking first.
If a bard does it, refuses to listen and basically doesn't care, petition him to get him suspended/banned.
The current server rules already consider swarm kitting (by default, even if the bard is alone in the zone), as zone disruption (
"Monopolizing/Engaging/Occupying most or all of the mobs in a non-raid area. This can be with or without intent to kill.") so if you have a problem, just petition to get him suspended/banned via the report forum (since logs need to be checked), just like you would someone who's training you on purpose.