To lock a viewport, highlight the viewport, Rt. Click, and choose Properties <Dialog Box>; Go to Misc <Heading>, then choose "Display Locked" or highlight the viewport, Rt. Click, and choose "Display Locked" from the sub-menu.
So many times [98%] I receive files from vendors who go thru the trouble of scaling their viewports without locking them. Just in case any one is unaware of the benefits of locking your viewports:
Locking your viewports will allow you to pan in your viewport without changing the scale or view of your viewport.
I did hear a rumor though, which I have not yet taken the time to discover: Irregular shaped viewports cannot be locked....?