Assume that Anselm's argument is valid - that it follows from the definition of "God" that God exists. In that case, couldn't we simply refuse to use the word "God" that Anselm has defined for us? If one of the premises of the argument is a definition, and definitions are constitutive of languages, isn't one rebuttal just not to speak an Anselmian language? If someone defined "phlogiston" so that it necessarily existed, she wouldn't thereby have a knock-down argument against the ontology of modern biology. She would have a linguistic quirk to be corrected or ignored.
A similar objection comes to mind in certain ethical and epistemological discussions, where it is argued that some substantive theory is just analytic. Someone might argue that it follows from the meanings of the words "will" and "good" that the object of ethical judgment is always the will, or that it follows from the meanings of the words "state" and "know" that we should state only what we know to be true. In response to this, can't we always simply refuse to use the relevant words with the meanings they are taken to have? Sometimes philosophers focus so much on the meanings and entailments associated with particular words because we're interested in better understanding the conceptual scheme we actually employ. But at some point, in these sorts of discussions of folk vocabulary, isn't it available to us to make new concepts? If we're clever enough, can't we cook up a "know" or a "good" that is better suited to our purposes than the "know" and "good" we have received from our ancestors?
I've assumed here that, at the crucial point, someone can't make a transcendental argument that we have to keep our folk vocabulary just as it is. But what form could such an argument take?