Are you talking about the bolt down under the seat that locks it on the seat so no one can just walk by and pull it?
I set a nut to lock it in place I think, lock tite works too I guess?
If you are talking about the adjustment bolt that adjust the angle .... I adjust it to suit the clothing I have on and I don't recall it loosening.
You can make a threaded bolt self lock by drilling a small hole across it out near the end and just before inserting it into the threaded hole, push a small piece of string trimmer line into the hole and letting the bol;t self trim it?