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.
The only people who would be impacted would be the activity farmers when they are trying to catch up with their potential activity. It will take them longer, but they will still catch up, nevertheless.