Delete all iMessage conversations. Compatible with macOS Monterey. Please check Reduce motion before launch this apple script.

delete_all_imessage_conversations.applescript

# System Preferences -> Accessibility -> Display -> Reduce motion

tell application "Messages" to activate

tell application "Messages"
	set chatCount to (count of chats)
end tell

tell application "System Events"
	tell process "Messages"
		set frontmost to true
		repeat chatCount times
			click menu item "Delete Conversation…" of menu "File" of menu bar 1
			delay 0.2
			click button "Delete" of sheet 1 of window 1
			delay 0.2
			try
				click button "Report Junk" of sheet 1 of window 1
				delay 0.2
			on error errMessage number errNumber
				
			end try
			click menu item "Go to Next Conversation" of menu "Window" of menu bar 1
			delay 0.2
		end repeat
	end tell
end tell

Very cool. This was the final pieced I needed on my own script. I hacked together AppleScript, shell scripts, and Python to export my contacts from the Contacts app, then search my contacts for each sender and delete them if they are not in my contacts but retain them if they are. Happy to share it.

It's kind of a mess. I wrote it to just clear my old chats and have not had a chance to "product-ize" it yet but anyone is welcome to use it. https://github.com/iconifyit/batch-delete-imessages