Rear sprocket bolts keep coming loose

Hi , I have a lexmoto street 125 , and have recently replaced the chain and rear sprocket however after a couple of days riding the bolts are coming loose , I've already replaced 1 that's literally fell out on 2 occasions , any ideas or tips to keep them locked in? I can't tighten them as much as I'd like because I struggle to get a spanner on the head of the bolt , Many thanks, John

 

you should have a lock tab of some sort to hold them in place

 

Spring locking washers should work or alternatively, get some medium strength thread locker from Halfrauds (sorry, Halfords).

 

What they both said^^^