Well, you could have all of the tournaments we part of a specific season. Then don't add the final 2 tournaments to the season. But the Stats tab doesn't allow you to "adjust" the values. Adjusting the values would be somewhat pointless anyway, since all of the data is derived from the tournaments themselves, so as soon as you changed the filter, or exited the program, the adjustments would be gone.
You'll have to do this manually, outside of the software, I think. Unless someone can come up with something I haven't thought of.