TMs can currently only be removed from the list one by one which means endless clicking when you have to remove them all.
Over a year now...
For sure this is a brilliant idea.
Or just select multiple TMs by holding Shift or Ctrl, and then remove them.
Try my PlugIn, it Has 'remove all' button for all TUs.
DL Trados Studio PlugIn - play with TM
-it is version 2017 Only
-it does not have proper SDL's signature, so warning window pops up when it starts
Would be a great help!