Creative Solutions

Eat them.

Shoot a leg off and put medical supplies on the opposite end of the room. If there's more than one of them, leave only enough to treat all but one of them.

That sort of punishment only works when the subjects have any sort of concern for each other. Otherwise, it is just an extra bit of competition.

Now, if the wound was less lethal that it appeared and the medical supplies were tainted with something toxic, you could have a good object lesson on your hands.

Crazy Goronak's is always looking for bodies then. Not sure how you feel about chemical weapons though.

Put them on a shuttle going through the Omega Four.

This one requires a somewhat large expenditure of resources and can take a good deal of time to come to completion, but it had a very impressive effect. Certain groups within Cerberus were rather fond of it. Develop a nasty, lengthy, and ultimately fatal pathogen, one that can infect any member of the traitor's species but only displays symptoms on those who are genetically related. For the best results, infect the traitor with the pathogen shortly before releasing him in a location not far from his family, so that he will be the vector that leads to his family's infection. If this is not possibly, covert deployments near their residences will have to suffice.

Why go to all this trouble, you ask? Making this part of your standard policy can serve as an effective treason deterrent. A good number of betrayals happen because somebody was threatening the traitor's family. This method of coercion will become much less effective if everyone knows your inevitable response.

Adding onto this make sure that he is the only traitor, you need to be thorough, don't want any weeds growing back. Once you have all known betrayers fingered I would recommend exile - into a thresher nest - naked and unarmed.

Also Blitz's idea is good. Cruel, but effective, but this also is assuming they have family, and that they actually care about them. As he stated it's costly and might even be more-so if you have to go through the trouble of looking up personal information on the person. During the rebellion on Revolution our forces often kidnapped and leveraged people against the government, if command felt they were better eliminated, we'd kill those close to them and more-often-than-not they'd come at us angry and stupid. One officer just kept throwing his soldiers at us without formulating any strategy beyond that, to the point that once enough of them had been terminated that they eventually turned on him for just mindlessly ordering them to rush our positions.

Pay a Batarian to stab them in a nightclub. Worked for me.

Betrayal usually warrants a boot out the airlock.

You stole that off the SIU.

The circumstance of the betrayal is the major factor, and I would employ the weapon they used against themselves. For instance, I once discovered that my fiancee had been sending nude holos of herself to her then-secret boyfriend.

So I sent them to her father.

Don't batarians just make slaves out of the people that annoy them?

Humans and turians are fond of the phrase "equal treatment". I'd employ it in your scenario, Mr. Seeker.

As an addendum, many of the posters in this thread could benefit from post-modernist studies. It might assist in reading comprehension.