I have used a 1/4" brass cap in place of the white plastic compression nut on a water filter when I abandoned a pipe just like yours - you could use a cap that matches whatever thread size your setup has. I know you can get PVC, Brass, and even stainless caps in various sizes...
Jim's suggestion of a piece of rubber under the hose clamp is a good option if it can sufficiently cover the hole - I have used a piece of bicycle inner tube under a hose clamp for a leak on a galvanized pipe before...
If you get a 7/16-20 NF tap with 25/64" drill bit, you can drill out the current hole (3/8" or 24/64") and tap in threads. Then use a stainless steel screw with teflon tape and thread sealant as a plug.
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