Just keep coming up with good dares and they will be back... Just because they have an account does not give them a reason to stay if the forum is filled with immature "Dare me, but don't dare me this" posts instead of actual dares to be used in the games they are hoping to use dares in...
Closing off the forum to members only will decrease Search Engine ranking across the web, so you would actually end up having less Guest to even consider signing up...
You could give people incentives for signing up and posting, they can use their points earned from posting to buy access to a special forum where pictures and videos ARE allowed. But the admin should probably start by enforcing the age minimum since he cannot build in a system to hide mature content from users as he could if he built his own system.
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