There is no p trap under our bath, it was missed being put in when the house was built. The house is on cement and the bath is cemented in. How can I stop the smell coming up with the plug is not in the bath.

There is no p trap under our bath, it was missed being put in when the house was built. The house is on cement and the bath is cemented in. How can I stop the smell coming up with the plug is not in the bath.
If it's cemented up how do you know you don't have a trap?
If there's no square hole in rhe cement behind the drain You have a major problem. Even with the drain closed off you'll still have a direct connecrion to the sewer through the overflow. Amd sewer gas will continue to enter you home.
There is no easy solution. The cement will have to be opened up and
A trap installed. If you need assistance click on back with some picturers. Good luck, tom

Tom raises an excellent point. I am going to continue one of his thoughts. Run some water and let it drain, just a gallon or so will do. Put a wet cloth or something over the drain hole since it sounds like you don't have a functioning plug. Do you smell sewer gas from the overflow? If not, you do have a trap.
If you determine, with certainty that you do not have a trap there is nothing that can be done other than having one installed.