You can lock/unlock any channel with [[Channel Bot]] (most have the bot), but I usually go to a channel I dedicated to #bot-spam.

Begin to type [[/lock]], and the bot commands will populate. Since we're trying to lock a channel, click [[Lock]].

Click [[\+2 options. ]]This will allow you to select the channel and for how long you want to lock it. You do not have to set a duration, but you must set a channel.

The free version of the bot only allows you to lock one channel at a time and you can set a timer for 24 hours or less. 

discord.com

This Scribe provides a brief tutorial on how to use Channel Bot to lock and unlock channels in your Discord server. This tutorial uses the free version of the bot.