I might have misunderstood. Is it a block of eight flats with say, two bins each? One recycling and other waste? Are they communal or does each owner have their own bins which are labelled etc?
Do they dump stuff in the other neighbour's bins? Have your neighbours mentioned its a problem? Can you not move your bins, furthest away, so it requires more effort to walk? Usually, people are lazy sods and will dump their rubbish in the nearest irrespective of owner, or type of bin.
They might be trying to wind you up. If they see you scrutinising contents; will think its funny and simply do it more.