Place all your viewports on a layer called "defpoints". The defpoints layer DOES NOT PLOT, even though you can still see it in your drawings. This layer was developed just for such purposes. This makes seeing the viewports possible without ever worrying about the frame plotting out.
If you don't have a layer called defpoints, don't make one. Rather, put a dimension on the drawing, then delete the dimension. Layer defpoints will be created for you. You can freeze layer defpoints so you don't see it on the drawing. No harm will come to your drawing.
The defpoints layer was created for the definition points of dimensions, but some users put viewports on this layer. In case you didn't know how not to plot a layer correctly, you would click "do no plot" on that layer in the Layer Properties Manager.