I want to delete list of sign-in names from dropdown menu on forum log-in page

I am a forum member. I click on the box to insert my username. A dropdown menu appears with several different options... misspellings, mistaken names for this particular forum...

How o I delete the list of the different usernames, leaving only the corect one there?

Please do NOT advise me to use Ctrl + alt + delete.

IT. Doesn't. WORK.

and whenever i hit the fn key, the screen 'shoots up' exposing the desktop! The fn key is ineffective for deletion of data!

How do I stop THAT happening?!