Temporary Toilet Bowl Seal ?

Problem is that you have a shared waste and so any problems would be down to yourself, landlord or council. The best thing to do, would be to have a meeting with the other tenants to discuss your concerns and see if collectively, you can all come to an agreement on things put down the WC. This is the only advise that I can give.

There is a fitting made by mcalpine its basically a rodent stop but it goes on the pan connector it has lever door that opens when you flush the toilet but shuts after the flow of water goes away and it will stop back flow. But if the waste is blocked already when you flush this could cause it to flow back when the the rodent trap opens

In the long term, the problem needs to be addressed as stated by the previous plumber. In the short term I would use a plastic bag and expanding foam. remove as much water from the toilet bowl as possible and insert the bag into the bend where the water usually sits in such a way that you can fill it with the expanding foam and then tie off the open end before the foam escapes. The foam will expand in the bag and seal the U bend, although I doubt that it will be 100% watertight. Obviously this is a "hack" but I imagine that its better than having your neighbours waste entering your property. Hope this helps, good luck.

Hi I agree with the answer given to the extent of asking other tenants to re-assess what they are putting down their toilet's. There are many more factors that you would need to consider. Firstly, the only way you can seal your toilet, is by removing it, and placing an expanding bung inside the waste pipe. It means you will not have a toilet to service your needs. ( highly not recommended ) 1. Is the current soil waste pipe suitable, and sufficient to service all the toilets. 2. How many bends are in the system, as these will restrict the natural flow. 3. You can try and use cling film over night. ( You will need a peg for your nose when you take it off in the morning )