Bottom bracket lockring keeps loosening up The lockring on the bottom bracket keeps loosening up, it's now ~15-20km. What can I do to prevent it?

The steeper the hills I ride the quicker it comes undone - it's a single speed so it's clear and I guess expected that the higher the forces the quicker it will loosen up.

UPDATE: I have lockring tool which I use on my commute rides which is wrong size. The lockring was tightened by bike mechanic 2 weeks ago using a proper tool, but it came loose within 30km.  

UPDATE 2: 
When the lockring becomes loose it causes a click in the BB. If I push the left pedal it will click and then if push the right pedal it will click again. In other words, if I put one of the pedals up I can make the BB click if push/pull the crank towards/away from the seat tube. 

UPDATE 3: After I removed the cranks and the locking, it turned out that the non-drive side BB cup is not tight in the BB thread. I really mean the cup, not the spindle. Even when the cup is fully screwed in (so the spindle turns freely but doesn't have any wobbles) I can move the cup up/down/left/right. It's a tiny movement, but it's observable and visible to a naked eye. It's as if the cup is of wrong side. The threads on the cup and inside the BB look ok, but I haven't cleaned them yet.

If I put locking on the removed cup it also isn't tight. It seems it fits, but there is tiny sideways movement.

I don't think this related but after removing the non-drive side cup I still cannot remove the spindle which I think is unusual.
If it's coming loose so soon after being actually tightened fully, then it's probably one of these 2 things:


The lockring threads are gone or in poor condition, probably suggesting they were that way before or as a result of the last time someone tightened it. The easy fix if this is all that's going on is a new lockring. Most bike shops should have a stray one. They're standard sized.
Some kind of binding or other bearing issue is going on inside the
left BB cup that's causing the spindle rotation to turn the entire cup in
the loosening direction. Normally how it's supposed to work is the
spindle is turning counterclockwise, which makes the ball bearings
orbit counterclockwise but individually spin clockwise, transferring
forces to the cup in its tightening direction. (Hence why ISO right
cups are reverse threaded, and also why Italian right cups are
notorious for coming unscrewed.) A retainer installed in the wrong
direction may be able to cause this, in addition to chewing up the
BB.