When you say eTrex Venture, I assume you mean the Venture Cx?
If so, the maps are stored on the microSD card, so you can take it out and put it in a card reader and delete any maps from it. Or just plug in the cable and put the eTrex in USB mass storage mode, then it will appear as a drive to delete the maps from.
Why do you need to delete the maps anyway? MapSource deletes all of the loaded maps whenever you load a new set, I assume other software works similarly.
Is MapInstall Garmin's own Mac OS software?