Apple Mail App :: "Erase Junk Mail" is greyed out I'm running OS X El Capitan, and have 3 Gmail accounts signed into it. For some reason, the Erase Junk Mail menu option is greyed out. How can I use this option?
Go to mail>preferences>junk mail>under when junk mail arrives make sure options #2 move it to the Junk mailbox is selected and this should fix your problem immediately. 

Let me know if you need any help, this is what worked for me after I let my junk mail build up to over 600, which was killing me, I can understand you want it gone! Should look like this: