Removing Watermarks

I have created a javascript to add a watermark to a PDF form when clicking a button.

Here is the code:

this.addWatermarkFromText({
cText: "NOT APPLICABLE",
cFont: font.HelvB,
aColor: color.ltGray,
nStart: 0,
nEnd: 0,
nOpacity: .5,
nScale: 1.75,
bOnTop: false,
nVertValue: 140,
nHorizValue: -140
});

However my problem is that when I RESET the form everything resets fine but I would like for it to remove the watermark as well.

Can someone tell me what javascript code I can use to remove this watermark??

Thanks for all your help.

Sarah