Currently, there isn't a mechanism to "delete" a cohort group. As long as a patient from the cohort still exists in Ocean, the cohort group will persist as well. Patients can only be assigned to one cohort grouping at a time.
If the patient list is re-uploaded again but assigned a new cohort name, Ocean will see that these patient references already exist on the site, and simply move them into this newer cohort group that has been created. Once the old cohort group hits zero patients, it will disappear.
Patients are auto-purged from the portal if there are no pending actions assigned to them (please refer to How long are patient records (with PHI) kept in Ocean? for more details). Eventually, if all patients attached to a cohort are purged out of Ocean, the cohort group will disappear.