Allowing unsportsmanslike jerks to play only one end without them being penalized is unfair.  I understand there may be times when something comes up and you have to run, but your points should take a hit anyway.  My suggestion to solve the problem of hit-and-run artists is this-- 

Mandate at least 2 ends to every game. If someone quits before finishing the second end, their opponent wins the second end 3-0. 

On the second topic of mid-round quitters, they should always lose 3-0 as well.  All too often players quit when they realize their opponents are going to win more than one point.  The common cheat here is to throw a stone in that temporarily gives the cheater an advantage; he/she then quits before the simple takeout can be thrown.  As a result, the quitter only loses 1-0 due to the poorly conceived formula.  He/she should always lose 3-0; even this rule would be generous at times for the cheater, but it would be a drastic improvement.