Having US govt. on your ass should a decent deterrent.
Just take a look at how hard FBI came down on cartels and individuals who were involved in killing Enrique Camarena. Cartel leaders were arrested in Mexico and several individual in the US.
The US cannot really send special forces there without risking a massive escalation.
But sending a spy to a software developers house and assassinating them probably isn't going to stop the problem - more people will spring up doing the same.
Russia has always been notoriously hard to spy on.
I would not be surprised if there was only a handful of well placed assets and most of the spying being done electronically.
Also, the US and allies can enforce Russian AML laws as written on paper. If, say, the UK freezes all of Oleg Deripaska's assets there, Vova will absolutely get the message. We're not going to bring down the Russian government with military force for a million different reasons, but doing it with sanctions and prosecution is a totally different story.