Hmmm...
assuming you are working with robotframework-excellibrary
That library is global in scope and looking at the code it will not let go of the file until you open a new workbook.
My first suggestion would be to either put the files in unique folders or stop trying to use the same file name over and over (e.g. work on a copy).
Reload Library in BuiltIn is no help here because it doesn't fully reload the library.
If you are too stubborn to do change how you store the file, than there is always another way ;)
You can re-initialize the state, freeing up the file by:
${lib} Get Library Instance ExcelLibrary
Call Method ${lib} __init__
This is a dirty hack, but it as clean as a dirty hack I can think of.
The file should be released shortly after (garbage collection needs to run). So you should retry Delete File with Wait Until Keyword Succeeds a couple of times.