How can I disable one switch in a three-way circuit?
To know for sure, we need to see pictures of the wiring and switches. But I'll make an educated guess:
Common (Shared) Hot
The two switches shared a common hot wire. One hot in, two switched hots out. There are three ways to connect one hot to two switches:

Two switches built together with a common hot
This is the easy way - essentially two switches in the space of one. I know you didn't have this, because if you did then there would be no way to remove one switch without removing the other switch. But listing it for completeness.

Pigtail Hots
This is the preferred method because it avoids the problem you found. The hot wire comes in to the box and is connected to two short wires, pigtails. Each pigtail connects to one of the switches.

Daisy Chain Hots
Hot wire comes in and connects to switch # 1. Another wire connects from switch # 1 to switch # 2. The two connections on switch # 1 could be two wires under one screw (not usually recommended, but often done anyway) or one wire under a screw and one wire in a backstab connection. If you disconnect all wires from switch # 1, then there is no longer any hot wire for switch # 2.


You need to look at the wires going to the remaining switch. You should have one wire going out of the box to the light and one wire going out of the box back towards the circuit breaker. What I suspect you will find is that one of the wires actually has the other end loose, but capped, in the box. That is your formerly-hot wire. The hard part is figuring out which wire from the removed switch is the hot wire. The easiest way to figure this out is a non-contact voltage tester:

With the breaker on, use the tester to identify the hot wire. Turn the breaker off and then connect that wire to the switch instead of the loose/capped wire.