RFQ market makers are never obligated to provide a quote. If they cannot provide a competitive price at a given moment, for example, they may decline to make an offer. Moreover, they may decline to respond for any reason at all. Because the taker address is always known to the market maker, the maker can easily monitor taker behavior over time. Makers can potentially identify arbitrageurs, by identifying specific taker addresses that seem to repeatedly take advantage of the maker's mispriced orders. And because the maker has the option to choose whether or not to respond to the quote request, they can easily blacklist taker addresses that they deem to be arbitrageurs.
Furthermore, the request sent from the 0x API to an RFQ market maker also includes the API key associated with the trusted integration client. In the same way that makers can blacklist taker addresses, they also have the option to blacklist specific applications that integrate with the 0x API.
Finally, with the RFQ-M system, market makers have last-look on any quotes they issue, so it is impossible for an arbitrageur to pick them off on stale quotes.
In order to retain access to RFQ liquidity, it's important for clients of the 0x API to be good citizens of the marketplace. For end users of trusted integrations, being a good citizen means actually filling the orders they claim to intend to fill. And for trusted integrations, it means properly managing their users' intentions; more specifically, it means adhering to the flow of using indicative quotes by default, and firm quotes only when appropriate. For market-makers using RFQ-M, it means building a track record of honoring quotes that have been outstanding a short amount of time.