Larsen Black Plastic Rain Cap for NMO Antenna Mount

If you don't want to remove the NMO mount, there are caps you can get to screw onto the existing threads. Personally that's the route I would go, that way you can always reuse the NMO mount if you decide you want a booster in the future, or get a GMRS radio, or your ham license. You can just attach to the existing coax, screw on the correct antenna, and you're good to go. If the mount itself is broken and you just want to completely remove it, there are hole plugs you can get as well. If you use one of those, I'd put a thin layer of RTV silicone on it so that you have a definite water-tight seal. Those results were found via a google search for "nmo cap" and "nmo hold plug", respectively. There are a lot of options like different shapes, and even stainless steel caps if you want silver instead of black, when it comes to the cap. There may be other options available for the plugs, too, you'll just have to google it yourself and see what you can find. Or take it to a radio install shop, they do this sort of thing every day for decommissioned police vehicles headed to auction and fleet vehicles having radio gear removed for lease returns. Anything beyond that you're going to have to get a body shop to patch the hole, then repaint.