If those emails “are showing online” in your Trash / Bin folder then close eM Client and “delete them online” and then reopen eM Client.
If those emails “are not showing online” in your trash / bin folder, then try running the manual database repair on eM Client which should then clear the trash emails that won’t delete.
To manually repair eM Client database for Windows, see @Gary post link below.
https://forum.emclient.com/t/item-cannot-be-delete-from-trash/95660/2
To manually repair eM Client database for Mac, do the following.
Close eM Client and then “open the Terminal app via either Launchpad or Utilities” on the Mac. Then Paste and run the following command.
/Applications/eM\ Client.app/Contents/MacOS/eM\ Client /dbrepair
Then reopen eM Client and see if they are gone.