[Geoff Bacon]

It sounds as though you are trying to use the eraser to remove some of the underlying file (probably a pdf). The eraser only removes bits of previous MSP annotations to the pdf e.g. one has scribbled freehand over the file but only want to clear some of it

To block out some of the pdf, select the fill tool (highlighter) and set both the outline and the fill colour to white.
Alternatively, if you are correcting the pdf using text, select the 'A' tool and set it's background colour to White (or whatever) - it typically is None (which means that the pdf underneath the text is not obscured).

Geoff