You have to create a new permission profile.
Admin --> Members --> Permissions --> Edit Profiles
In the
Profile Name field, set the name for your new permission profile. In the
Copy Permissions From field, select "Default". Click on Create and the new profile will be created. Now, click on the new profile. Different types of users will have different permissions inside the profile. Click on the Modify button in the
Regular Members line. If the view mode is set to Simple, change it to Classic. Untick
Post new topics,
Post replies to topics: Own topic and
Post replies to topics: Any topic. Click on Save Changes.
Now, go to Board Permissions.
Admin --> Members --> Permissions --> Board Permissions
On the left of every line, you can see the board's name. On the right, you can see the permission profile that the board is currently using. Click on the board that you'd like user's to have read only rights and in the
Permission Profile field, select the newly created permission profile. In the
Allowed Groups section, make sure that
Regular Members and the post count based groups are ticked (moderators should also probably be ticked, guests... per your liking). Afterwards, click the Modify button.
Done

. That board now has read only privileges for users

. Repeat the second part for every other board that you'd like to have read only privileges for regular users

.