Hi,
The easiest way to go about this is to hide the fields in ArcMap before you check data out. (On the fields tab of the layer's properties window, just uncheck the boxes you don't want.) You'll notice that during the checkout process, on the window where you specify where the folder is saved and what the ArcPad Map should be called, there is a checkbox that says "Only get fields selected as visible in layer's properties".
This will prevent those fields from even being checked out to the unit, which will probably improve its performance anyway, although obviously you can't alter any values in those fields.
Hope this helps,
Eric