"For fastest response, please use basecamp for all project related communication"
Then just respond to basecamp communications quickly and emails slowly. This isn't up to your clients, and there doesn't need to be a 'hint'. Just start a consistent pattern of not responding to email requests for at least 24 hours, maybe 48, and responding to basecamp activity quickly.
Your problem will be solved, although some clients might balk at this. Those clients may not be able to be rehabilitated - but in reality they are self-identifying as crappy clients.
Welcome to your first step up on the client ladder! Good clients like process, bad clients like to bug you all day with urgent matters.