Good news folks!
I have taken your feedback into consideration, and have implemented a 'protected MilPoints' system. The balance you see won't change, but any points you purchase can't be taken away by another user assessing your posts.
For those who want to understand the inner workings:
- MilPoints earned by normal means are added to your "regular" balance
- All purchased MilPoints are added to a "protected" balance
- The balance you see and use shows the total of your "regular" and "protected" MilPoints
- MilPoints deducted when another user assesses your posts will never be removed from your protected balance
- MilPoints spent on anything else will be deducted from your protected balance first; when your protected balance is gone, points will be deducted from your regular balance
- If you spend more points than you have in your protected balance, your protected balance will be set to zero, and any remainder will be deducted from your regular balance
- If your regular balance goes negative while you have a protected balance, only your protected balance will be shown
- As soon as your protected balance hits zero, your regular (negative) balance will be shown again
If you find that confusing, join the club.

I had to draw out all the possible combinations and check them carefully to make sure the new system made sense... I believe it does, but if anyone has questions or finds a flaw in the logic, please let me know.
Thanks again for the useful feedback,
Cheers
Mike