I'm having a slight difficulty understanding your question, but taking a guess - you can arrange to delete the Trash folder in the following way:
Whilst in Mail > Go into Mail Preferences (Shortcut is Command plus comma) > Click on the Account in question > Mailbox Behaviours > Under "Trash Mailbox" you will see "Erase deleted messages" > Click on the exposure triangles which give a number of Options as per my screenshot: "Never", "After One day" "After one Week", "After one month", "When Quitting Mail". You can do this for each account.
Under On My Mac are a list of Mailboxes and they have to be linked to an email account or several accounts - and as you say, your are getting Trash Into the Trash Mailbox which appears to serve all accounts.. You are doing this - it doesn't do it by itself. Your list of Trash Mailboxes looks like this:
What you asked was how to empty the Trash Box - this can be manually or a daily, weekly, monthly basis or when Quitting mail, as I showed you.