We use MapPoint extensively in our VFP commercial applications and have found that the "cannot quit message" is usually related to object reference issues of one kind or another. We have also noted that in some cases you will need to issue "THIS.activemap.saved = .T." before the close of the map in order for it to release properly. You may also need to play with the VFP.AUTOYIELD parameter setting (depending on what you're doing).
Hope This Helps
Trevor