I can understand your concern, but because you need a username and password for loggin into Tableau Public I don't think this danger is any bigger dan open shares in Dropbox, Google Drive etc.
Tableau doesn't have an option to disable publishing to Public; maybe a sophisticated rule in you firewall might be able to block uploads without restricting access to Tableau Public itself.
Johan de Groot
Yes. I did consider that the risk is not much more than other open shares; however, the chief difference is that Tableau Public and Tableau Desktop are products of very similar nature designed and facilitated by one company. Does it make sense for each end user to write firewall rules or to have a fix designed, rolled-out and supported centrally? Even if it's only a design change not to have publishing to Tableau public being in the same location of the interface as publishing to Tableau Online, then I think that would at least lessen the risk from a human error perspective. As of now if one has a public account, that account can be saved as the default publication route for online publishing.