You don't deal with it. Simply, you owe your ex husband nothing. If he's contacting you about it, block his number or ignore his calls. Your kids are adults now & it's not your responsibility to support them anymore. In saying that, it's your choice if you want to help them. If your college age kids need help and you can help them financially then it's your choice to do so. I too have 2 kids in college (divorced for a year now, my ex tried to make me pay 1/2 tuition but by law I'm not responsible. And it's not that I don't want to help my kids, I do what I can, I offer & if they ask me I give what I can afford)
If he's throwing you under the bus (assuming he's doing that thru your kids) then they are old enough for you to have a mature conversation with them. You don't have to go into the gory details but explain your position in that your ex makes a lot more money than you & financially you can't contribute as much as he can however if they need support/help with school expenses you will do what you can, all they have to do is come to you and as you- NOT your ex!