Only one way to delete POI one by one in Nuvi!
Or you also can use a Garmin software like BaseCamp or Mapsource or a third party utilities for editing the copy of current.gpx. Than you can copy the renamed version of your current.gpx (for example mypoi.gpx or temp.gpx) as you did with Lidl.gpx to device after clearing all POI in it.
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