I believe you can lock threads in some of the ACNH boards (like Island Journals and the trading boards), as well as in the Museum and TBT Marketplace, by clicking the 3 dots next to "Watch" at the top of the thread and then "Lock thread". if it doesn't have that option then you'll have to ask a staff member to manually lock it for you. hope that helps!
It’s very rare that a staff will lock a thread. I’ve asked about locking some of my threads and it didn’t happen because there wasn’t really a reason for it. I just thought the topics were a bit controversial and wanted to say goodbye to them. You can ask, but I cannot guarantee the threads you want to be locked will actually be locked.