Here's the solution, prevent people from using In-Progress for Frontlines and it's all fixed. Why you even have that option is beyond me. Now you have people aiming for a quick win/rewards.

I'm sure the amount of actual trolling done is minimal, and it's more the amount of people thinking it all takes too long or having run out of time by the time the queue finally fills up, thus leaving.

You cant punish 71 people that may have other ingame plans or statics because of one bad apple leaving over and over. If you absolutely -have- to implement something, make it so that you cannot leave a queue more than once a minute.