If you’re trying to reclaim some disk space, years of old messages and associated attachments can easily consume several gigabytes. I recently tried to find out how to delete my old iMessages… unfortunately for some reason Apple makes this more difficult than it should be, without any obvious ways to do it.
The first thing to do is to close the Messages app. Then delete the contents of the Messages folder:
I restarted my computer and all the old messages were now gone. However, a few weeks later while doing a spotlight search, I noticed that a few old conversations began to show up in these searches. Odd, I thought I had deleted everything. I later found out that older messages seem to be archived elsewhere. I also deleted the contents of the following folder, and the messages were now gone.