Once cleared to land the runway is yours.
If that clearance is cancelled the runway is no longer yours.
Issuing a new clearance cancels the previous.
Simple.
If ATC issue a clearance to land and then have to send you round because of a vehicle on the runway, ATC have an MOR to write so taking Captains Authority and landing anyway will just add to the number of words on the form but could change the investigation from incident to accident.
Regards,
DFC