It's an old house and recently during a renovation, I realized that both sides waste stacks are sharing a shaft touching one another and the neighbor's one was cracked, practically watering my side with sewer every time they flush their toilet.

No products aside from bathroom tissue may be flushed down the toilet.

I also get that my current mindset is probably overly negative, so I don't want to be sending that out into the universe and driving people away as my first impression)I am also realistic about what I offer (in the form of investor) is really just money and in 2020 everyone is just flush with money and infinite lines of credit at 3.5%, so it's not like it would have been in 2008 or so.

The toilet has to flush and the kitchen sink has to work.3.

Unless you can prove which tenant it was and it was in the lease that these items should not be flushed, you are SOL. 

Since the average trip charge is $85-$100, this covers the stupid requests... like when the kid flushed a toy down the toilet and clogged it.  

You just flushed $3,500 down the toilet! 

As for the septic that could cause unique proportys in a rental, if your tennent doesnt know better they could really mess it up flushing things that dont degrade or kill the bacteria in the system by dumping household chemicals down the drain. 

And BTW it will only be heated for awhile after each flush.  

That’s like throwing your cards in when your holding a royal flush.