I'm a native Russian speaker.
The first thing that comes to mind when I hear "флуд" is the meaning of "idle, meaningless talk". The concept is similar to "flaming", but it's milder, does not necessarily imply bad intentions (perhaps the person doing it simply likes to talk a lot) and can be used humorously to denote any kind of prolonged Internet banter.
Likewise, "флу́дер" is online slang for "chatterbox", a person who writes many messages without bringing any value to the the conversation.
While the term may have originated from the name of a DDoS attack, that is not the first meaning that comes to mind when people hear that word and not many are even aware of it.
The word on its own is NOT abusive or offensive and I would strongly suggest to exercise extreme caution banning people merely for using it.