After talking with Tableau, this is not possible with the current version of the TABCMD utility.
I've come across this issue as well.
Version 8.2 will allow for deletion via the REST API, however in the meantime my workaround was using the following:
tabcmd createsiteusers "users.csv" -t "MySiteID" --license "unlicensed"
createsiteusers will actually modify an existing site user, and unlicensing them will effectively prevent the user from being able to do anything on that site.