To delete existing transaction payees, first go to MoneyWiz Settings by clicking on the gear icon in the menu. Once you're in Settings view, click on Payees section.
To delete a payee, swipe left on it and click on the Delete button. MoneyWiz will ask you if you are sure.
If you proceed, the payee will be deleted and removed from any transactions that might be using it. MoneyWiz does not check whether the payee is used or not so please check this yourself before deleting the payee (for example by generating a payee report for all time period).