Limiting network access for individual applications, on a per-WIFI-SSID basis?
Use ipfw to bind a rule to your specific interface, blocking the site in question outright. For example, if CrashPlan's IP is 224.2.2.4, the rule would write as follows:

ipfw deny ip from 224.2.2.4 to me in via en0


The "ip" tag blocks all traffic (tcp and udp, alternatively, you can set either or if you like). The IP address can be set to a range for more efficacy (e.g., 224.2.2.0/24). The "me" tag is self-explanatory. Alternatively, you can actually set that up to a specific IP address too. So if your home network assigns you 172.16.1.4 and tethering assigns you 168.128.1.5, then you can further ensure you only block the site when tethered, and not all the time. The "in" tag deals with direction of the traffic (in or out). And "via en0" deals with binding it to a specific interface.

You can also set ipfw to launch during boot, so you don't have to run this manually all the time. It's not a trivial process but does require some cli. Alternatively, you could check out WaterRoof. It's a free ipfw front-end that pretty much automates the entire thing. A terrific ipfw primer and setup assistant.