presume you mean the external services lever, not the main 3 point lift lever.
on mine theres a movable stop, ive got two ,so i can lock on either of the two external connections , one way for the front loader and the other way for the hedge cutter.
i have on other tractors used a small door hinge bolted to the quadrant to lock the lever when required.
just fix one half of the hinge down so the free half will flip over to hold the lever. when the hinge is closed the lever is free to pass it.