Silencing somebody doesn't need to involve sending him a legally binding gag order. Nor does it necessarily require killing him.

There are lots of carrots (e.g. job offer, lucrative contract) and whips (e.g. a threat to ruin his business or professional reputation) that a government agency can use to persuade someone, even a foreigner, to keep something a secret for a certain length of time.

This obviously won't work on someone who is under Putin's protection, for example, but then we're talking about cyberwar, not a lone white-hat.