Most common questions related to adsense are How to apply for adsense ?
How to apply adsense ads in my blog or website?
How to create adsense ad units ?
How to delete adsense ad units from dashboard ?
In this case according to the Google Product Forums you cannot delete once you created an ad unit but You can HIDE your unused ad unit, by clicking on Hide button under Actions.If an ad unit is hidden and idle for a while, Google may automatically remove it from your dashboard.
You could however rename it and reuse the ad unit elsewhere.
Source: Google Product Forums
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