You can archive a team to make all channels and files of that team read only. You will need to be a team owner in order to archive or restore a team.

Archiving a team.

First select the settings cog at the bottom of your teams list. This will bring you to a list of your teams. 

On the team you'd like to archive, click the three dots on the right, then select 'Archive Team'

This will then ask if you'd like to make the associated SharePoint site read only. Selecting this box will make any files in the team read-only for members. Team owners will still be able to modify these files.

Teams will then archive the team. This will cause it to be hidden from the sidebar, and appear in a separate section in the full list of teams.

Restoring a team.

If you'd like to restore a previously archived team, first select the settings cog at the bottom of the teams page.

You will then need to expand the archived section, then find the team you wish to restore. From here, you can click the three dots on the right, and then 'Restore Team'