How to minimize unwanted net usage by processes to a trickle?

I have this problem sometimes of what seems like excessive Inteet bandwidth usage. Sometimes I have to use a metered connection, so the issue becomes more important. Leaving aside things like malware (which may happen, and for which I do have anti-malware software installed), what are some ways to reduce Inteet usage (other than that directly initiated by the user) to a trickle, so as not to waste bandwidth and money? Thanks in advance for all answers.

I'll mention one way that I am exploring: NetBalancer for Windows. A friend also told me about nethogs for Linux. Not tried yet, but will.

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