Users and Groups
Banning is largely pointless if they're spambots. You might also find the load of the query to remove multiple accounts and their posts is too big. You'll have to do small groups of less than 100. Maybe even less than 20.
It's better to just stop those bots registering in the first place.
Please visit Preventing Spam in phpBB3
for various options for fighting spam in phpBB.