On comment spam

We tried almost any known solution to stop spam in our forum; each one provided some "resting time", until spammers found a workaround and began spamming us again. At the end, the only solution that has worked for us is as simple as limiting the number of links that can be included in each post. It is a little nuisance for users, but most of them do not need to include ten links on each post; however, spammers soon lost all interest in our forum, once it became useless to them.