How hard is it to chapter a Soldier with an approved retirement date?

Depends. How close is the retirement date? Chapter packets, sadly, can take quite a spell. Took about 2 years for 2 of my former Soldiers to be chaptered for APFT failure

Its going to be tough, and if you are his/her commander then you are going to have to decide if the time and effort is worth it. Unless it is some majorly egregious act - its really not worth the time or effort. I would agree with Maj Reams, leave him alone; most of the time the folks that have approved retirement dates seem to slip into retired on active duty and start coming in late or start to slack on their appearance and you are just going to have to instruct them to maintain the minimums for the good of the unit until they are out. I made it a practice to ask myself, is he/she just retired on active duty or are they just trying to press my buttons? With some it was both, but for the most part it was road - and I needed to keep my ego in check.

Now, why on earth would you want to chapter a soldier who intends to retire and has an approved date? Unless this troop is a total dirtbag, with a stack of disciplinary actions, this seems awfully petty. Do you have a valid reason to deny the troop his/her pension? If not, let him/her retire in peace, and go out honorably.

Unless it's a capital crime or something heinous, concentrate on the overachievers and those who are eligible for early promotion or have earned solid decs. Recognize those that deserve it and let this one go.

Just curious, what would be reason enough for trying to chapter a SM who has a set retirement date? If there is not a CM case pending, what would you consider grounds to negate 18+ years of service with no pension?

If you are just researching and couldn't find the answer, then you must obviously have someone in mind. Otherwise, one wouldn't be wasting one's time looking into such an area or bring it to a forum for one, for this is called beating around the bush hoping someone will give you an idea or an avenue to use, this is why the IG is emplaced to answer your question or questions and as far as to what actions or enaction you can do.
Or is this for a taunt to hang over someone's last days of service, in which in many cases I have always found out it was just a matter of disliking someone and making them miserable during their time they have left.
I know from my own experience of someone trying to riff me out of service before my retirement date, due to their personal dislike of me. Because I had a number of Art. 15's in my junior enlisted years of service. Hey, I was a wild child.

Initiating a Adverse Action, such as a Chapter 11 or 13 that would hold up a Retirement, requires actual proof and facts, that substantiate the Chapter's criteria, to be presented at a Hearing. Said SM has the right to contest said Chapter action with countering proof and facts, and to do so with either JAG or Civilian Counsel. From personal experience I had a Commanding Officer that, for some reason, took a dislike to me and sent me on a Command Referral to the Division's Psychiatrist for a Fitness exam. When I got there the Psych' asked me why I was there and I told him, It took exactly 2 phone call's to have the Division Surgeon come in, and right behind him, the IG's SGM, hear what I had to say, and had me stay there pending. 3 hours later the Division Surgeon sent me back, AND MADE IT VERY CLEAR TO LET HIM KNOW IF ANY OF THIS WAS GOING ON ELSEWHERE. When I came back the C.O. had left, on a 'Leave', and never came back. Using Chapter's and threats of Command Referral's-for-Separation are both stupid and show a serious lack of Command awareness. It also reeks of Command Abuse, and more practically, a very public seen settling of grudge's and vendetta's, both that are not going to look good on an OER, much less a Formal Inspector General Report of Investigation.

I came up against this problem right before retirement(19) years. Command came after me because I took a “Soldier’s Stand” against racism. I faced a CM or retire. They let me retire peacefully. But I really wanted to soldier on. So, it can happen…you can loose your retirement depending on the circumstances.

how about, don't be a richard. There is a reason you can't find it

I am curious as to what lead you to this question. This doesn't seem like a question out of "the ether". But the answer is pretty clear from the responses.