Use this code as the field's custom calculation script:
var oSigState = this.getField("Signature17").signatureInfo().status; event.target.display = (oSigState==0) ? display.hidden : display.visible;
The problem with this sort of thing is you're making a change to the document after it's been signed, which will require an additional save and it changes will be indicated when validating the signature.
Filling in fields is a common thing to do after a signature field is signed, so I would recommend setting the field value (to "URGENT"/blank) rather than showing/hiding the field.
I decided to go with a check box instead:
if (this.getField("Check Box2").value !="Off")