Currently the signature added by Foxit Reader from going under "Protect"->"PDF Sign" can not be removed.
Two workarounds:
1) In Foxit Reader, you may go under "Comment"->"Rectangle"->drag a rectangle around the signature then right-click on it to open "Properties" ->"Appearance"->change fill color to white;
2) Or you may install Foxit PhantomPDF Business, then go under "Protect"->"Mark for Redaction" then drag around the signature then open " Properties" to change the fill color and apply.
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