How's that again?
When I read that description, it sounds to me that you want the text to adjust for the merge fields but then you don't want them to adjust for the merge fields because the text overlaps a container field or layout object with the signature. Don't see how you can have that both ways!
But after a few seconds thought, perhaps this is what you have:
Body with layout text including merge fields
Graphic Object or container field with signature image in the Body
If so, use part set up to add a trailing grand summary part and move the signature into the trailing grand summary. Set the Layout text block in the body to "slide up" and "resize enclosing part" to remove unused space between the signature and body of the layout when the layout is previewed or printed if such is desirable for the results that you want.
Or you may want to put that signature in the Footer of your layout...