Enter Alexagate. This is a $100 device that fits on top of most of Amazon’s Alexa speakers. The idea is that it emits ultrasonic frequencies so that Alexa can’t hear what you’re saying. If you do want to summon Alexa, just tap three time on top of the device to pause the ultrasonic frequency, and you will be able to use your smart speaker just fine.
As the frequency used is beyond the range of human hearing, you will not be able to hear it which means that it will not interfere with your day to day activities. Of course, this might kind of negate the entire reason of having a smart speaker in the first place, but if you’re someone who truly values their privacy, it might be worth taking a look at.
If you want to get your hands on Alexagate, you can head on over to their website for the details on how to buy it. For now it seems to only work with Amazon’s Echo devices, so for other digital assistants or speakers, you’re out of luck, at least for now.