Hey, this can be done but it's not straightforward (took me some time to figure out :). Magic trick is to remove zimbraMailQuota attribute. But you have to supply current quota for it to work.
So you can first parse it (eg. from zmprov ga) or set it to some predefined value, like so:
Code:
zmprov ma user@domain zimbraMailQuota 0
zmprov ma user@domain -zimbraMailQuota 0
HTH