On your iPhone or iPad, open Settings
Tap your Apple ID name at the top
Tap iCloud
Tap Messages
Turn off Use on this iPhone
When prompted, choose Disable and Download Messages or Delete from This iPhone
Open the Messages app
Delete the message threads you want removed
Go to Recently Deleted in Messages if available
Select the conversations or messages
Tap Delete
On a Mac, open Messages
Delete the conversations you want removed
In Messages, go to Settings or Preferences
Turn off Messages in iCloud if needed
On iCloud.com, sign in with your Apple ID
Check whether Messages are available for deletion in your account settings
Delete any synced message content from all devices signed in to the same Apple ID
Empty Recently Deleted on each device if available
Restart devices to ensure changes sync
Verify Messages in iCloud is turned off if you want future messages to stop syncing