As some experience shows  , accident happens when people are authorized to push the code themselves (after diff review or not ...).
It may be times to discuss some contribution policy adaptations.
- (current) contributor can push once the diff is reviewed and accepted.
- contributor never pushes. Staff member pushes once the diff is reviewed 
Please feel free to amend the description ;)
 We'll then have to discuss the git adaptations needed (git commit message format, merge policy, etc...).