There's no simple way to delete clients from the dashboard. However, they will transition out of the dashboard if they haven't been seen in 30 days. You can also give them a 'blocked' policy in the meantime to ensure that they can't access anything if they reappear.
However there was a mention that something like it might be possible leveraging GPDR privacy features
Solved: Remove/Clear Client - The Meraki Community
Not sure of manually adding a client with the same name but different MAC (I can't see why it wouldn't work) but you can definitely re-name discovered clients to have the same name.