I have had to deal with almost the same thing!
I renovated my 1870 Creole townhouse about two years ago. While the plumbing in the house is new, the city sewer system is not. Locals here in New Orleans know not to flush anything but waste and toilet paper because we know what nastiness can and will happen if you do. However, tourists seem to believe that this doesn't apply to them.
In the first 6 months of hosting, I had to call in a plumber 4 times (once on a national holiday on a Satuday!). At first I put up a cute little sign across from the toilet that said, "Please do not flush paper towels, feminine products, wipes, hopes & dreams down the toilet!" After the 2nd time it happened I added a house rule about toilet use, then I printed and posted the rules in various places. It kept happening until I changed the rule to this-
Do NOT flush anything other than waste and toilet paper down the toilet- no feminine products or baby/facial wipes. If a plumber has to be called during your stay or on the day of checkout and it is determined that a blockage occurred starting from the 2nd floor, you will be responsible for the repair costs.
Since then, no isssues! Not one!
You might want to create a similar rule stating that there is an additional $100 (or something) charge for any septic issues that occur during a stay or within 24 hours of check out. If you have multiple guests, you might not be able to pinpoint the exact culprit, but perhaps the threat of having to pay extra will be enough motivation for guests to finally start following the rules.
Good luck!