How to determine which process or application is using excessive data
You can monitor the traffic with numerous applications and tools. One that is already installed is 

/usr/bin/nettop


To block this unwanted traffic the easiest you also have numerous options:

– block the destinations via /etc/hosts

– block the ports used by those unruly programs using your builtin pf firewall 

– use one of the application level firewalls like
–– RadioSilence
–– LittleSnitch
–– HandsOff
–– Vallum
–– TripMode

Most important and practical is of course to just uninstall the offenders if posssible.