I use my phone’s tethering all the time– on Caltrain, in airports, out at cafes, and anywhere I can’t get good wifi– and frequently worry that Dropbox or some other syncing software is going to decide to download gigabytes of data from some shared folder, burning through my data allowance and leaving me with a huge bill. I currently handle this by manually turning off anything with a sync component every time I tether, but it’s an annoying (and human-fallible) process.
Based on the connection type and the network, Little Snitch lets you selectively block traffic from certain applications. Not only does this solve the excess data problem, it also helps with security– if you have an app running that’s not sending over TLS, you should block it on insecure wifi networks to prevent being sniffed.
Little Snitch comes with a full-featured, non-expiring demo, and costs 35 bucks. A little pricey compared to most utilities in the App Store, but it works incredibly well, and in the scheme of things, probably pays for itself in one 10gb backblaze job over Verizon LTE.