I am sorry but currently there is no way to reset the quota globally/user specific via the UI. We have a feature request for this, so hopefully it will be possible in the future.
I have a large group of users (several thousand) who want individual quotas - the solution is to set HUGE global quota amounts and store a quota value in the users AD account. This is read (LDAP) and their usage calculated as Quota.Volume SizePerMonth - Quota.Volume.RemainingMonth.
This is compares to the AD amount to trigger the block.
If AD quota is increased, then takes only a couple on min for the user to be "free" to continue.