Sewer line sabotage

We have a sewer system that has a lift station. The last man hole before the lift station is getting clogged up by items such as wash cloths, tampons, syringes etc. We are unable to trace the items to a specific home as it is not getting caught up in the individual service line. We have considered sending a letter to residents reminding them that the only thing that should be put down the drain is toilet paper, and including a warning of a fine/ eviction, but we are concerned it might give others the idea to sabotage the sewer line. We are having the mainline rooter almost every two weeks right now because of this problem. We are going to start using a product called GreaseX in the last manhole to see if this helps unbind these items. I can not think of any way of identifying the culprits and not sure if a letter will help. Anyone have any ideas?