Locking a channel allows people to view previous messages, but prevents them from sending new ones (within the channel).
To lock a channel, you will first need to navigate to your "Edit Channel" menu by right clicking the channel you wish to lock and pressing the "Edit Channel" button, as pictured:
Next, navigate to the "Permissions" menu by clicking "Permissions" on the left of the newly opened menu.
Enable the "Read Messages" permission, and disable the "Send Messages" permission, like so:
This channel is now locked. The channel will be locked to any person or role that does not have these permissions for the channel changed.