I should imagine you would go to the Manage Domains page and click the edit key under the web hosting column for the appropriate domain/sub-domain and then untick the Google box before resaving the changes.
Deleting the domain/sub-domain if you no longer need it would have the effect of removing Google …
Opinions are my own views, not DreamHosts’.
I am NOT a DreamHost employee OK! :@
Act on my advice at your own risk!