there exists a tool to open manhole covers, and should such a tool fall into the hands of an unscrupulous individual......
Maybe you know already but locks requiring a special "tool" like a pentagonal wrench, are not able to exclude isiots, it does slow em down . Often ,because folks who dump trash into infrastructure like waste or storm drains can merely be given some sort of impediments, which is often good enough to deter them.
I don't recall any cities even in some of our hillbilly cities (like Franklin or Charleston W Va) where people actually seek out sewers to clog with junk. We usually have trash receptacles and dumpsters so people either use stealth methods or they drive way out to the country and dump their junk along roadsides.
People are pigs all over. I cannot claim any "advanced sense of aesthetics" that prevents us from dumping junk but it seems like you folks are actually hunting out sewer culverts in which to jam that really annoying trash, we just litter the countryside in hopes that hiway crews will pick it up .
I imgine that the first time someone knows they've got a pooh plug is when the road floods.