just being yelled at by tenant for a bunch of drain flies that cannot be killed

[Kelly B]

I am a landlord of a single family house in New Jersey. For the last two months, I've been battling some drain flies in this property. I called several pest control companies and they basically told me upfront that they cannot get rid of drain flies since they come from drains. I still hired one (horizon) that costed $200 and sprayed the house, just show my tenant that I am working on it. I also had plumbers (twice) to come in and did leak detection on my drain and the plumbers claimed that they didn't find any leaks. Basically I think I did all I could and have no idea where to go next. I also bought different products and asked my tenant to pour down in the drain, as recommended by plumbers and pest control company.

The plumber says there might be a hole in the foundation as it's an old slab house, and I don't know what to do about it.

My tenant just called and yelled at me for not being able to solve the problem for two months, and threatened me that he would take matters into his own hands. In the past, he had charged me over $200 for a drain stoppage, and $350 for changing filter on the furnace.

Could someone recommend what I should do next? I searched on the internet for drain flies and tried almost every chemical that claimed to kill them.

Should I offer some credit to the tenant once for all? I sincerely would like to take care of this issue for him but I just don't know how.