I would suggest finding some boards that should not count towards activity, as I would theorize that the worse of signature spammers both farm accounts (eg collecting potential activity by collecting activity posts by making a single post every two weeks), and spam the forum with paid signatures.
Another potential solution would be to make it so you need more then a single post made in a two week period in order to collect the (potential) activity points. It could either be setup so that you need to make at least 4 (or some other number equal to or less then 14) posts in order to collect the 14 activity posts, or so that you both need to make at least 2 (or some other number equal to or less then 14) posts and be online at least 2 hours (or some other amount of time) during that activity period in order to collect the activity points. Yes time online can easily be faked, however it is at least a small hurdle for people who are spamming the forum in order to collect potential activity points.
We should also set a limit of posts counted towards the activity. For example, people can post as many constructive post as they want, but only 6 posts a day can be counted towards the activity.