Does deactivating a pending authorization work? To be honest, I haven't tried that one yet. The recommended way is to trigger a validation attempt so that the authorization goes to a success or fail state - see the last section of the rate limiting policy:
If you have a large number of pending authorization objects and are getting a rate limiting error, you can trigger a validation attempt for those authorization objects by submitting a JWS-signed POST to one of its challenges, as described in the ACME spec. The pending authorization objects are represented by URLs of the form https://acme-v01.api.letsencrypt.org/acme/authz/XYZ, and should show up in your client logs. Note that it doesn’t matter whether validation succeeds or fails. Either will take the authorization out of ‘pending’ state. If you do not have logs containing the relevant authorization URLs, you need to wait for the rate limit to expire. As described above, there is a sliding window, so this may take less than a week depending on your pattern of issuance.
I'm not aware of a way to get all pending authz in case you don't have all of them in your logs. In that case, waiting for their expiration or using a new account are probably the only options.