To delete iMessages on your Mac, you can follow these steps:

1. Open the Messages app on your Mac.
2. Select the conversation you want to delete messages from.
3. Right-click (or double-click on a MacBook) on any message.
4. Hit the Delete option to get rid of it.

If you want to delete all iMessages from your Mac, you can follow these steps:

1. Open the Messaging app on your Mac.
2. Locate the message you’d like to delete.
3. Right-click (control + click) on the message and then click Delete.

You can also use third-party software like MacClean to scan and remove deleted iMessages from your privacy history. If you enable Messages in iCloud, deleting iMessages on Mac will also delete them from your other devices with the same Apple ID.