soon we will add the ability for the community owners to let users bypass challenges if they have X amount of karma or account age.

You should look into POW captchas like mcaptcha or powcaptcha. It's a far better user experience.

the challenge exchange between the authors and community owners are arbitrary, they can be anything the community owner can code and that the author client understands, so POW is possible, but I'm not sure it would stop spam properly, a server can do a lot POW for cheap. Maybe combined with another challenge.