This is not accurate.
Your activity is minimum of 14*number of two-week periods in which you have been active and number of your posts. So, if you delete some of your posts and your post count decreases to 100, your activity wouldn't be necessarily 100. Your activity will be a number between 0 and 100.
Thanks for fixing me there was a calculation error only which I fixed otherwise whatever I said was accurate. In order to earn 1 activity point a user has to make at least 1 post on the forum but the number of activity a user can gain is limited to 14 each two week period. And if we keep that thing in mind and suppose that a user who has been active on forum for at least 20 weeks and has made at least 14 or more posts per week during those 20 weeks then we can say that the user has most probably got at least 120 activity points or more to get eligible for full-member rank.
Now suppose that the user has made at least 200 posts during those 20 weeks and now he/she somehow decides to delete some of the old posts. If the same member deletes at least 100 posts from the 200 posts and now he/she has 100 posts on the forum. That's an indicator that the user has only 100 posts and in such case the user will have 100 activity points and he/she is no more eligible for full-member rank even if he/she has more than 100 merit points.