There is a fairly easy way to delete the entire GEDCOM that you imported -- and believe me, you aren't the only one who's ever done that!
Have you already linked folks from the GEDCOM to your family?If so, then unlink them.
Use the Search/Find function (binoculars icon) to find someone in the unwnted GEDCOM.Then click on the "Advanced" feature, and in the Relationship Filter select "All Related."This should find all 15,000 folks in the GEDCOM -- assuming they are all related. Then click the "Delete" button.
If you don't think all 15,000 folks in the unwanted GEDCOM are related, then you can delete a block of RIN numbers.This is easier if you haven't added a whole bunch of RINS after you imported the GEDCOM.
Sort your file by RIN (Search/Find function; select "RIN" instead of "Alphabetical").Record the beginning & ending RINS of the folks in the imported GEDCOM.Then go back to "Alphabetical" Search/Find, use the "Advanced" feature and select "All" in the Relationship Filter.In the Field Filter, select "RIN" and enter the range of RINS filled by your unwanted GEDCOM.Then click on the "Delete" button.
I've imported quite a few GEDCOMS over the years.I have created a temporary PAF file (named "Temp.PAF") that I import the entire GEDCOM into.Then, from Temp.PAF, I select just those I want -- all the descendants of a couple, for example, plus the parents of spouses; or all the ancestors of someone.These I export to a new GEDCOM, which, in turn, I import into my main PAF file & link with the folks already there.
Hope this helps?
-- Cliff in Virginia William C. Barney President, Barney Family Historical Assn.