okay mate, first things first, we need to figure out where the actual problem lies. if you raise the back end of the bike and spin the wheel (take the chain off when you do this to allow for maximum freedom and little friction) does the wheel spin freely or is there constant contact between the disc and pads??
if there is no touch between the pads and disc i would look into the brake lever spring. if the spring is soft it will take very little pressing power to lock the back wheel, of course the stiffer the spring, the more feel you have and thus your ability to control the back brake will be increased 10 fold. of course the other option is the check your oil levels, of course as you would know if your brake canister has too much oil pressure is increased and thus slight touches will cause a great ammount of pressure on the brake pistons and cause lock up's, you could try to remove some oil from the fill up canister (only if the oil line is above maximum level line)
try those things but first before you play find he source of the problem as it will help you and us alot better to find the approperiate resolve to the issue.
Cheers, Tony.
