Any way of achieving this?
Hi – unfortunately, no. You’d need to modify the code to add a new permission to restrict this ability.
And, since a moderator can lock or unlock topics, they would still be able to reply to a locked topic anyway by simply unlocking it.
Building out your membership site can be a lot of work. Easy Membership Sites has pre-configured all the Simple:Press plugins and tools you need to build a complete membership site in a fraction of the usual time. Check them out: Easy Membership Sites