I think (can not guarantee) that the following set up will solve your issue:

Go to Configuration > Reward Points > Advanced Reward Point Calculation Rules and click Edit. In the Module drop-down box select ot_coupon. In the Action drop-down box next to it select Subtract and click the green cross button to its right. Then click Update and do some test orders with or without using coupons.

Please post back whether this solves the problem or not for other readers to benefit. Good luck!