leave it half-compressed for a couple days. will reduce the power (not an exact science though).
Really, best way is to just change the spring. Adding padding has little-no effect on actual power. I have 1/4" of sorbothane that I added to my cylinder head... didn't change my fps at all.
No, leaving a spring compressed at all will not reduce the strength of any quality spring... if it did, your car's suspension would be shit after a few days. Someone in the UK literally tested for a full year, the FPS and Joule output of several springs he left compressed in a custom jig and saw no difference from month to month. The only time he saw a difference was immediately after a new spring was installed, it would settle, but after it had settled it was consistent for the remainder of that year. My G&G M14 Veteran's stock spring was a consistent 383 w/ .25's for nearly 150,000 rounds and I never saw so much as a change in that FPS of more than +/-2 FPS.
Honestly, the best way to reduce FPS is to change the spring.
Also: Adding sorbo to my G&G M14 veteran dropped it from 383 w/ .25's to 405 w/ .20's. 509mm Barrel and full cylinder. 6mm worth of Sorbo. Do the math, reduced volume in cylinder is going to yeild a drop in FPS unless your cylinder is already massively over volumed compared to the barrel (IE: Full cylinder in 363mm length barrel).