Over the last day or so we have had either two or more people DDos'ing the minecraft server that we run, we have tried to ban their nicknames and i.p addresses, but it doesn't seem to keep them banned (i'm going to guess that they're usiny Dynamic I.P's)

Everytime they've crashed the server it's been whilst other people are ingame, not whilst the server is empty, the BOT that we use then crashes, and it fails to restart for some time before being manually restarted by our server owner, we're rather stumped by this and not sure what to do, any suggestions?

Not sure what else to add to this and i also realise this isn't a lot of information to go on either, but we are desperate.