I think you can click on a circle you're not interested in hearing from (they're listed on the right-hand side in the current/old UI) and there will be a slider at the top of the stream from that circle that will allow you to completely mute that circle.

Then I guess you can add your own "topic alerts" by simply doing a search for haskell, lisp, scheme or whatever at the top and then click "Save this Search" to permanently add it below the "What's hot" button on the right (I don't think there's a way to limit that search to one of your circled though... maybe there is I'm not sure).

EDIT: You can limit a search to all your circles, but not to individual circles. Then again it is unlikely that your "Gardening" circle will have much to say about #haskell.