Well I have better idea means Vbulletin has software which allows user to ban from specific thread not whole forum.
It is better because whenever you ban someone they can read every forum/thread depending on the permissions in their user group or the forum permissions - when they're not logged in. Only IP address bans can prevent someone from reading the entire content on your form.
But still Ban by IP won't work in all cases.
So Just Ban the users from specific threads like this He can see the people's work but won't able to download it.
I know that next question will be how to identify a person like this who comes with dummy id then still you can ban every doubtful user from threads like these and allow them to PM Admin for their identity.
It still has errors but it would be still better than to ban user from whole forum by his ID or IP.