There's a few ways strewn about Legends and Canon.

The most common way is through masking yourself/force signature. Many of the Rule of Two Sith like Plageuis, Palpatine, and Dooku did this so they could go about their daily lives and not be immediately sensed by the Jedi. However it wasn't entirely foolproof, unless you could produce a faux "light-side" aura like Darth Zannah would. Generally the force would be hazy, clouded or absent, and that would give off warning signs.

On a more physical level, Ysalamiri evolved to not be sensed through the force by their natural predators the Vornskr. This wasn't just cloaking their force signature, and was instead a bubble that they could share with other Ysalamiri to create large pockets on their homeworld where the force was repelled. While you couldn't be sensed in this bubble, anyone reaching out with the force would probably still find the bubble itself since there would be a notable void or nothingness in the vicinity of it. It also came with the downside that you yourself could not use the force should you enter the bubble, though if you were already not a force user like Grand Admiral Thrawn, this wouldn't be an issue for you.

The last example that I could find would be from the Yuuzhan Vong. The Vong were cut off from the force after the destruction of their homeworld caused them as a species so much pain that it was stripped from them. This meant that they couldn't be sensed or even touched by the force itself. Though there were notable workarounds. Instead of pushing the Vong into an object you could still throw physical things at them. You couldn't lift the Vong and drag him to you, but you could suspend the air around them and bring them to you that way. You might not be able to sense the Vong, but instead you could tell it was them when you felt the void in the force that they left behind. Some Jedi were even able to develop a type of "Vongsense," after spending enough time around them and their biotech, though this was very rare. Something very similar happened to the ancient Rakata species, but that was due to a plague that wiped out their connection to the force.

In all 3 examples however, you can still tell that something is there, you just can't tell specifically what it is. Either it would become a haze, or just a plain void in the force. Which would be much more concerning to me, what's so awful that it can't be touched by the force? Or what's so evil that it's trying to hide itself in the force?