Lots of people have serious smoke sensitivities: I know, I’m one of them. I walk into a house that someone has smoked in and within a few seconds, I can’t breathe.
Unfortunately, unless you spelled out the importance to you of taking possession of a smoke-free house in your offer to purchase, there really isn’t much you can do.
First of all, until they actually turn over possession of the home, the sellers are free to smoke inside and outside. After all, it’s their home, and a “man’s home is his castle.” Smoking in one’s residence is not illegal, and unless you contractually had them warrant the home was smoke-free, they are well within their rights.
You are entitled to possession of a home in the same condition it was in when you viewed it, but I don’t think you’d be able to argue that the condition of the home had changed dramatically unless there was recent damage caused by smoke and that’s unlikely. To be honest, I suspect the owners had smoked in the bathroom ensuite long before you decided to buy the house and that you weren’t in there long enough to find out. It’s not a room where we tend to do more than stick our head inside during a viewing.
So, what can you do?
Nothing contractually, I’m afraid. But you can clean non-porous surfaces with a variety of cleaners, including vinegar, to try to remove the smoke residue. On painted surfaces, Kilz is a product that people swear by when it comes to removing smoke film as well. Once things warm up, keep a window open and keep the fan going, and eventually that smell should go away.