Deleting previous file names dialog box when saving files ..

Not sure if Ive described this correctly but I would like to clear the
previous file names dialog box when Im saving files, its almost like
autocomplete, but it picks up on any previous file names and tries to use
them again, how do i remove this.

So if I start with the letter "a" it will show all the previous file names I
saved as with the letter "a" and so on, how do I clear it, or remove it.
Thnaks