Electrical - 50 amp abandoned wiring from hot tub connected to breaker (outside)
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.