I would strongly recommend you upgrading ideally to 7.2.2
The new version now enjoys fairly standard benefits of captcha and email verification which should cut out 99% of any spam.
I get very little spam out of the 30,000 posts a month - maybe 1 or 2 posts are spam if that.
Spammer use false email addresses and if they can't verify their address and you do not allow guests to post, then you will see very few issues.
Of course there are other security reasons why you should upgrade, and needless to say software companies would not discuss such matters on an open forum due to a potential risk to other software owners.
