If you have a discussion with a potential donor, that end without a donation, inactivate the major gift opportunity. You can then still see that that you have had the discussion and see in what step the discussions was halted. You can easily re-activate an inactivated major gift opportunity.
However, if you add a major gift opportunity by mistake, go ahead and delete the major gift opportunity. If you want to restore a deleted major gift opportunity, contact Mira Live.