How about limiting the number of accounts that are created every day or lets say each hour?

I am not sure if anyone has this data but there must be numerous accounts that are being created ever day. Out of these accounts only countable once could be from genuine or brand new user whose visiting the forum for first time. Some of them could be come back users with lost/hacked accounts.

. . . and most of them might be with the intentions of spamming.

Limiting account creation will slow down the burst spamming and might give more time to moderators and other users to fight against spam.

It won't stop the spamming but it will slow down the process.