Clip Cell

Did the cell come from a MicroStation cell library? If so, you could obtain this library and edit the cell there and then perform a replace.

Using The Replace Cells Tool

If you don't have the original cell library, I would think the only other option would be to drop the cell if you wanted edit components of it.

the cell came from a add-on program, GuideSign.

Is there any reason you couldn't attach the cell's model from the cell library as a reference attachment then clip as you would any other reference file? Or put the GuideSign created cell in its own model within your DGN then attach that model as a reference and clip? If you want/need to do this to more than one or two cells then I'd copy, drop, edit, and create my own cell and replace like Andrew suggests. 

A less elegant solution could be to draw an opaque shape using the background color which would mask what you wouldn't want to see. 

Matthew Eiben said:

Or put the GuideSign created cell in its own model within your DGN then attach that model as a reference and clip?

I've done this many times. Not only does it allow me to clip the fabrication specification when I need and easily lay out the sheets of details, I can also use it to place the sign on the plan sheets when needed. With the "Treat Attachment as Element" setting, it behaves exactly like a cell.