Strikes me that you have two different problems with two different kinds of bandit (as we would call them this side of the water).

The first - the improver who hasn't had their handicap changed - is easy to deal with; the league should be able to apply an adjustment based on their league results.

The second lot - who throw away shots at the end - are more difficult because they are in effect cheats. My answer would be not to play the team with the worst offender - give them a walk over and explain to your fellow players why.