Thank you for your help. All mentioned bannings and deletions have been taken care of.
The issue with locking pages is, unless there's a super-intelligent filter able to detect vandals/spammers and prevent them from editing (which is unlikely), we can't flip the switch for the entire Wikia database to be only accessible for editing by administrators. Not all registered users are vandals; some can be contributive!
However, the only effective and feasible option we could switch the Wikia to is the option of preventing IP edits, which, I believe, is already put into effect.