How to remove a co-author from an article published on arXiv?
Yes, technically one can modify the authors of an arXiv article. See the replace instructions on arXiv. See also this answer on Quora. But, as far as I understand these instructions, the previous version with the initial co-authors will remain on the server.

Note that it was no good scientific practice to initially associate a co-author to the paper that didn't contribute to it. Regaring the claiming of the authorship by X after removing his name from the paper, I have no answer.