There may be a supplier available in your invoice allocation dropdown menu that you would prefer to mark as inactive (such as 'ABC 1' pictured below).
To do this, you will first need to click into the Company Contacts page, located on the left hand side of your company's homepage.
Once you are on the Supplier page, search the name of the supplier, then click on the name.
This will then bring you to the General Supplier Settings page. To mark the supplier as invisible, simply click on the 'Is Visible' switch to turn off that setting.
Once that setting has been switched off, scroll down to the bottom of the page, and click the Save Changes button, and allow a moment for the change to save.
The next time you go to your invoice supplier account dropdown menu, that supplier will be greyed out, and you will not be able to select it!