So if I understand correctly, You are referring to someone whom you know personally on this site that you can identify as for sure it is this actual person.
Then you say he/she has been convicted of a crime. And then you know that he/she has not repented of this sin and is in fact now trying to be a better person by congregating with fellow LDS members around the world. You say that you are uncomfortable with this person being online.
This is called judgment. This person is going to live their life as they see fit. I suggest you do the same. I am surrounded in this church by people who can't let the sins of others go. Especially the small ones. (not suggesting that this crime is small.)
The idea that you wouldn't be able to interact with the good people on this site because of another person on this site tells me more about you that the other person. (read prodigal son). You can't let the actions of others allow yourself to be poisoned with the pride of judgment. That is Satan's greatest tool. If he can get us to look at others instead of our own.
From your statement this person is already convicted of a crime. Unless this person is now trying to commit another I would ignore them. If you have some proof that this person is doing something inappropriate at home or church go to the authorities, but this sounds liike a case of "I can't believe this" scenario.
Most of the comments on this post surprised me. Calling cops, identifying someone for allegedly no being themselves? uh hummm. Kind of scary. No one on this page is using their real name. I doubt a lot of you would want to be exposed because someone else thinks you should be on here. Just some thoughts.