This already has been released even on NulledBB plugins section..
https://community.mybb.com/thread-180082.html
1.8.x
Lock until reply [Modification plugin] 2.0
Submitted by The Professor, 21-10-2017, 05:17 PM, Thread ID: 52308
Thread Closed
RE: Lock until reply [Modification plugin] 2.0 21-10-2017, 06:13 PM #2
21-10-2017, 06:13 PM
#2