I have 50 amp wiring sticking up in the deck from a hot tub that was removed. The breaker is outside that it is connected to, how do I remove or terminate with a box and cover?
If you're going to leave the breaker, you could just shut it off and/or disconnect the wires connected to it. Installing a lockout on the breaker, would prevent anybody from turning it on. You should be able to buy a lockout in the electrical department of your local big box home improvement store for less than $10, though you may have to supply your own padlock.
If you want to install a box to terminate the wires in, you'll want to get a weatherproof box. If the wires are in conduit, get a box with an appropriately sized knockout. If the wires are in a cable, get a weatherproof bushing. Mount the box in a convenient place, and feed the wires into it. If the wires are in a cable, don't forget to secure the cable near the box.
Once the wires are in the box, install a twist-on wire connector on the end of each wire. Then push the wires back into the box, and install a weatherproof cover on the box.