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Imagine the following situation. A criminal committed a crime (for example a robbery) with several associates. Then he found out that his associates may have been investigated and arrested and they might have informed on him. So he took action and killed his associates to silence them.

Is there any idiom or collocation used for this action - for killing fellow criminals? I think it may exist because I heard in on a TV years ago but I was not able to find it.

EDIT: I found out that the detective in the show used the idiom tie up (some/a few) loose ends, which is quite general and can be used on plenty of others occasions. Answers presented below may be more suitable to the story.

I can't think of a word that's used exclusively used for criminals killing other criminals to prevent them from testifying, but there are several words and phrases that are often used in such contexts.

For example, there's the phrasal verb rub out, which is slang for murdering someone – but I don't think that's reserved for rubbing out a fellow criminal.

The word silence can be used in this context as well. As a verb, silence can mean "keep from expression, for example by threats or pressure." For example, a New York Post bioarticle says:

Hoffa was notorious for Mafia ties, corruption and violence.

Hoffa needed somebody who could use muscle to silence enemies. He told Sheeran, “I heard you paint houses.” Sheeran replied yes and added, “I also do my own plumbing” – meaning, he disposed of the bodies too.

The phrasal verb paint houses could work as well; according to the Online Slang Dictionary, it means "to kill or murder someone," with the phrase being a colorful reference to the blood that splattered on the walls as a result.

One other term I've heard used is cement shoes, which refers to using cement around someone's feet so that their body can be dropped into the ocean and remain unfound.

But these terms can be used for any act of murder and are not necessarily restricted to criminal-on-criminal violence and intimidation.

The phrase Cleaning House usually refers to getting rid of undesirable or corrupt members of a given group.

Edit: It has to be the person's own group, you don't Clean House in a group you don't belong to.

It doesn't specifically refer to criminals, for example a corrupt police department might "Clean House" by firing all the people who were willing to accept bribes.

However it can also fit your scenario as getting rid of people who would be willing to talk to the police (ie: everyone else)

"We found another body in the river, it's Stabby Mc-neckerson! That's
the fifth one this week"

"Mobboss Mcgee must be Cleaning house"

There is a proverb:

Dead men tell no tales.

That would be a preventative measure, if that's what you mean by silencing them.

An informal/colloquial idiomatic phrase describing an act of murder that has been planned in advance, usually in an "underworld" context, where for example a possible informant or a rival is "taken out of the picture":

He bumped them off.

Or if he employed someone else to do it for him:

He had them bumped off.

It's often referred to as silencing a witness. It's a euphemism for killing them (or in some cases coercing them) to make them "silent". Webster: to compel or reduce to silence.

Tying up loose ends

Means to murder those with knowledge of crimes you have committed.

Backstabbing

To backstab your allies is to betray people who trust you. Sometimes literally stabbing them in the back with a knife.

The implication is underhandedness and deceit, merely eliminating the competition would not be backstabbing. It relies on the victim expecting to be able to trust the backstabber.

"That filthy Backstabbing swine! We pulled off the heist and then he turned his gun on us and demanded our shares! We were supposed to split the money! Truly there is no honour among thieves."

There is also a quote from Benjamin Franklin's Poor Richard's Almanac:

Three may keep a secret, if two of them are dead.

This is more a description of the motivation for criminals to kill their co-conspirators, rather than a description of the killing, but does come up in those circumstances. It's often paraphrased to two can keep a secret, if one of them is dead or whatever numbers work for the particular criminal gang.

There probably is no expression that means clearly "to kill one's criminal partners", mainly because a major, perhaps the major, function of criminal argot is to conceal the real purpose of a conversation. Thus if the planners are overheard---say by wiretap, hacked email, or the maid---they can have plausible deniability: "We never said that! We said we had to erase the kids' crayon marks when we cleaned the house!" That's why all these expressions are so ambiguous, have so many confusing meanings.

OTOH, the opponents of the criminals might have good reason to be very clear about it and yet the cops seem to talk like that, too. Maybe it's an in-group thing.