Can one prevent a co-author from publishing on arXiv without consent?
Once an author has put their work into the public domain, it is very difficult to rescind it. All of the authors still have rights to the works, like attribution or trademark, but not copyright since that was given up as part of uploading to arXiv. So it might be impossible to force arXiv's hand with any legal tool where both of you are co-authors of the works. You do have one option though, but it's not pleasant...

If the co-author claimed joint-ownership over some part of the paper that you can prove (or they will agree) that they do not have rights to, then the works was illegally shared with arXiv, and arXiv have a duty to take it down if you DMCA it. The key is that you have to be sole owner of the copyright, and not shared ownership. Since you never personally signed off on the publication, that should be fairly easy to put forward. I don't know what arXiv says about this though, since their may be a fee for the co-author for retracting the paper - which I suppose you could both share, and i think you should, since arguably you should have done more to prevent a communications error like this ever arising in the first place.

Finally, may i just add, that this would not be illegal, but it would go against the principles of academic honesty...