Just practice slowly and work on playing clean. Then work your way up. Accuracy is better than speed... People would rather hear a clean, slow solo than a fast, sloppy one.... or atleast I know I would. You may be pulling your finger off the string so fast that you're causing it to make the sound of a pull-off. If you try to control your finger a little better when pulling them off... allow a brief moment for the string to quit vibrating, then remove your finger. Once you practice this a lot you will do it naturally.