You object to your lawyer's proof of claim (if he files one) or to confirmation of your X's plan (if he provides for the claim in the plan). But if you're the only creditor, the case might be subject to dismissal for bad faith. Seeking dismissal might be better for you, because the property settlement obligation is potentially dischargeable in chapter 13 -- meaning you might get pennies on the dollar if the case continues.
I'm not sure you can do anything to "remove a creditor" from someone else's bankruptcy filing. But, I'm not sure why you really care. If your attorney gets paid some money from your ex's bankrutpcy plan then there is that much left for you to pay directly yourself. It seem to me that it is more beneficial to you if he pays a part of the balance then if you must pay the entire amount.
File your own proof of claim. Chapter 13 creditors only get paid based on the proof of claim that are filed. If your lawyer also files a proof of claim, there will be an objection to paying this debt twice & the court will decide who the Trustee should pay.
Hope this perspective helps!