Setting a Fixed Scale (Locking the Map Scale)
Often it is desirable to set the Map Scale, and have it stay set... (ArcMap isn't always very good and returning exactly to the same map scale when you return to previous extent, etc. See also tips on using Spatial Bookmarks).
Of course you can set the scale manually, and can re-set it as needed. This allows zooming in and out (but the user has to be careful to remember to reset the scale...)
To Fix (set or lock) the Scale, open the Layer Properties sheet
On the Data Frame tab, click the Fixed Scale box and enter the the desired scale
Once Fixed, note that the Zoom tools and the option to manually set the scale are deactivated. You can still Pan, but the scale is locked.
This is true in the Layout View as well