I believe that you are looking at a parts adjustment. This is just an educated guess from the information you have given me.
If you go to jdpc.deere.com and do a search for 650 you will find a parts listing for your tractor. I like the sectional index but you can look at others if you like. Open the sectional index. Scroll down to hydraulics and open that. What you are looking for is section 126- Rock shaft control and linkage. Open that up and you will find a parts diagram for your tractor. Look for parts numbers 32,31 and 12. part number 32 is dished washers which indicates a spring type action. If you carefully tighten up numbers 31 and 12 which are two nuts you should be able to get your lever so that it doesn't move when you don't want it to.
Part numbers 20,21,22,24,and 15 are the three point hitch stop I was telling you about earlier. If you set that to the point you want your three point hitch to stop at that will be a temporary fix so that you can get your mowing done today.