Hi Isaac,
If you catch them, ask them politely not to do that, and be positive about it and positive about the apartment community when talking to them.
You don't want to create ill will over someone doing something they perceive as cleanly.
Just tell them it doesn't leave enough room for trash bags inside and that creates a mess for everyone.
Then end it on a positive note by asking them if they have any complaints about the property. There may be other things going on that need you attention that you don't know about.
If you have a monthly community newsletter type of thing you send out, put in there (every month) "Please do not throw couches and the like in the dumpsters it leaves no room for trash bags, etc...".
During your lease signing include a separate colored sheet of paper with large print on it with the same words as the monthly newsletter. Not a part of the lease, because I doubt you want to evict for this, just a sheet of paper.
Then when your maintenance techs see the dumpsters filled this way, have them fix it immediately.
If the furniture is in decent shape some people will run ads for it on Craig list "Free Stuff" and say it's by the dumpster come pick it up.
Just my thoughts.
Good Luck!