How to prevent user point totals from going below a threshold for a given user?
Here the solution with custom module implementation:

implement hook_userpoints($op, $params) function in your custom module to check the user points in every transaction with two option called points before and points after.

For your thing check if the points are less than your threshold value, set your threshold value after the transaction.

for more referencehttp://api.worldempire.ch/api/userpoints/functions?page=1 check this.