How to avoid OS X downloading things while using tethering [duplicate]

Solution 1:

TripMode

TripMode works on a whitelist system and blocks entire apps/processes from accessing the internet.

It is advertised with these relevant features:

When TripMode is ON, it prevents all your Mac apps from accessing the Internet but those that have been whitelisted by yourself. It typically stops automatic updates, online backups, Photos syncs, and various obscure apps from consuming precious data. Both uploads and downloads are blocked.

Track your data usage. See the data used per app, session, day, and month. Spot the data hungry apps.

New: You set the limit Set your data limit. All traffic stops when you reach it. Never go over your plan again!

Solution 2:

Little Snitch

Little Snitch lets you restrict specific network traffic from apps and processes. You can configure it to switch profiles automatically depending on the network that it is connected through.

You can set up Little Snitch to prompt on new connections, or configure the connections before the access attempt.