Moderators can be able to unlock the thread, but a thread of 21 pages long will not continue with useful posts but spamming, unlocking it will not be the best option, it is better the thread remain locked.
Are all topics locked by admins unable to be reopened by the user? I guess I remember having a topic that was locked by the administrator and I reopened it but I'm not sure. 
In general and in order not to lock your topic, it is better to be self-modulate to ensure that spamming does not occur. I don't visit the gambling section so I'm not sure if it's possible to create these topics or not.