Well I've done that several times including setting and deleting a new signature. I've fiddled with the font size too, because my old signature was size 1 and I think the default is size 2. Several times I've set a signature, saved it, then went back and either (1) put the cursor at the end of the line and used backspace until it would backspace no more, or (2) used ctrl-A to select the entire box, then delete key. Neither worked for me.
Is there some other way I could have deleted the invisible formatting? Is it something a user can do or do you need an admin account?
Let me know if there's any way a member can do this without assistance. Or otherwise next time I'll just PM you my request.
It was the font=1 formatting, right?