You can set whether the result of a Geoprocessing tool gets added to the map (if that's not already set in the Geoprocessing options in Pro). One of the members of the ExecuteToolAsync method is the GPExecuteToolFlags enumeration. You can use AddToHistory or None so it won't be added to the map. This is an example where I'm creating a feature class but not adding it to the map.
IGPResult result = await Geoprocessing.ExecuteToolAsync("management.CreateFeatureclass", Geoprocessing.MakeValueArray(arguments.ToArray()), null, null, null, GPExecuteToolFlags.None);