Good day. The permission View threads by others
should achieve what you are wanting; if they can't view threads by others then they can't reply.
I have a couple of add-ons that take that permission a step or two further, they are paid add-ons, but it sounds like just setting/revoking that permission above will accomplish what you are inquiring about.
Guests can post new threads in a forum if you grant them the permission. You can also set the option to allow posting before registering (though the guest would then need to fill out the registration form after they post their message for it to be actually posted).