A just released a new version of AVRFuses for Mac (Cocoa) that fixes a fairly glaring bug. The previous version would not let you unset fuses that had a single option. So, for things like setting the clock (where there are many options) it worked fine but for things like setting Serial Programming Enabled (which is just on or off) it would not work.
Get the new version at AVRFuses 1.1 for Mac