i dont use decopmpression to start the bike either.. i start the bike on the center stand and decompression isn't needed. But it does help when pedal-starting while actually pedaling the bike down or up the driveway.
i thought about permanantly closing the valve hole in the head .. maybe just welding it up nice and smooth. But the valve seals well and i prefer to leave well enough alone until i have a reason to change things..
Clean the valve's head and the valve seat of any carbon so it can't leak .. Attach a spring / lever or whatever to the valve stem to keep the valve pulled in closed. The spring should be strong. Look at any moped manual for a description of what the common decompression valve mechanism should look like.
A bolt or wire or other jury-rigging might loosen up.. and you don't want a compression leak. A spring would be best, imo.