I am looking for an answer to this issue also.
I use many forums in several classes online and want students to be able to return to read them but not reply or create a new post after certain dates. The current method of removing the permission is quite time consuming.
Thanks in advance for your help.
Just adding a link to the relevant docs for anyone who finds the question in future -number 12 in Forum FAQ ( I don't think it's that time consuming unless you have a lot that you want to lock)