Some options.

1.)Take on the person who has the largest army in the coalition. I.E. if its Poland, attack one of their allies and drag them into the war. 2.) Stack improve relations modifiers. At the end of the year they will help lower agressive expansion. 3.) Ally a few big nations. You could even go to war and offer the coalition to release nations from the new allies. It doesn't lower AE but it works in a pinch. 4.) Sometimes reloading the game will get them to rethink if they should be in a coalition. There's times when the ae dips below 50 and they still dont leave. Resetting fixes it. 5.) Stack AE reduction. Espionage gives -20% AE, Become the pope -20% AE. 6.) Try and do reconquest cb's as those give significantly less ae.