Phpbb im getting spammed

made a nice phpbb forum for my friend’s mw3 clan
its getting spammed real bad
we have capchas on
i keep banning the accounts and ips but they keep making new accounts
its sucking up all the bandwidth we’ve paid for
is there anything i can do to stop this?

Have you kept it up to date?

One of the downsides of third party platforms is that any vulnerabilities are well known to those who exploit it. The best way to prevent it is to keep it up to date.