To hide the page bottom guideline:

If you don't want to change the page size by dragging then go to the Page Size tab in the Options dialog and turn on "Lock page size".

If you do want to change the page size by dragging but don't want to see the page guide then you can disable it in the registry setting this value to 0:
HKEY_CURRENT_USER\Software\Xara\XtremePro\8.1\Opti ons\Rendering\ShowPageEdgeMarker

(Replace the "XtremePro\8.1" with the program and version you are running.)

Usual disclaimer: Be very careful when editing the registry by hand. You do so at your own risk.

Phil