Google Mail Added Graphic

[Tubular]

Before trying the below, make sure to have a backup copy of your 
original file.  Never save the file until you are sure you have the 
desired result.  Now for the fun part:

To remove the "branding" by Google Mail added to Calc spreadsheet, 
change the font color to white, hi-lite the columns / rows where Google 
Mail inserted its graphic. 

Next select Edit / Delete . . . remove all options for Delete except 
"Objects" click delete, change font color back.  Lastly save your file. 

Personally, I would select another name just in case Google keeps a 
record and will add its graphic to your document again.