What does it mean to be 'unable to sin' when in the glorified state?

I think this is something theologians have missed. It also pertains to the so-called problem of evil.

What does it mean to be 'unable to sin' when in the glorified state?

It may very well mean this: (I'll use the Ten Commandments to illustrate)...

In a glorified body we won't sin along the lines of the first table of the law (the first four commandments) because the very fact we are glorified means we are consummated believers, so the first four commandments having to do with God we will follow from our very glorified being.

Regarding the 5th commandment to honor our parents.....we won't have parents. No marriage, no children.

Regarding the 6th commandment to not kill.....you can't harm a glorified body. I can't kill you, you can't kill me.

Regarding the 7th commandment to not commit adultery......there is no marriage hence no adultery.

Regarding the 8th commandment to not steal.....in the Kingdom of God, glorified, we will have all that God has hence theft will be meaningless. If I have infinity of gold and you steal 40 pounds of it I still have infinity of gold. Knock yourself out.

Regarding the 9th commandment to not give false witness..... I suspect that in a glorified body everybody can see what each is thinking, so lying will not be possible. In other words what you are on the inside will manifest on the outside.

Regarding the 10th commandment to not covet others belongings.....since we all will have everything we need or want already coveting will be meaningless. Or coveting will become something positive like wanting to emulate something higher than you, and we will have the ability to create anything for ourselves.

Thus, unable to sin in a glorified body.

As for the so-called problem of evil... Evil can only occur because we don't have glorified bodies. Adam fell, didn't get glorified, nature fell, so we all have these flesh bodies that are subject to moral and natural evil. Evil becomes kind of funny when you look at it that way. "I can cut you and hurt you because you have a fallen flesh body, ha ha!" "I can drop a bomb on you!!" 

When seen this way the terror of evil is really just the Satanic element. He wants us scared by how he can hurt our bodies and steal our possessions. In the big picture evil is a much more insignificant thing. It causes suffering, true, but there's suffering before glory. So from God's point-of-view evil is not what it seems to us. It's part of His sovereign decree. It has meaning in the plan of redemption for each individual. 

Tell the atheists they're getting too hysterical over the fact of evil. 

If you're an atheist your suffering is meaningless. If you're a believer your suffering is redemptive and meaningful. Tough it out. God says He doesn't give us what we can't handle.

Higher aspects of time and level of being also play into this, but no need to go into that here...