I agree with Scot, it sounds like a short sale. Perhaps the sellers figure they got away without making a house payment for several, several months so they might as well continue with their unethical behavior and string along the buyers. Unfortunately, your'e making payments for your house and they are reaping the benefits.
I would immediately begin the eviction process, which typically starts with a 3 day notice. Visit your nearest Superior Court and start the process right away because it takes time to complete. At the same time, negotiate with the sellers who are currently occupying your house and costing you money. Find out their reasoning and motivation. Do they just need a little more time? Like stated before, if you can afford it, offer them some assistance. But make sure you don't pay them till after they have cleared out completely.
So...where is the listing agent? Where is your agent? Are they working together to get the sellers to abide by their contract? They should be working together to get these people out.
Check out the link below for information on evictions in Riverside:
Good luck and please update us here on this forum so we know how it turns out. We often provide advice and never hear back the outcome.
Keller Williams Realty
What is your agents involvement in this. If you have an agent, I would call their broker immediately and find out how they allowed you to close escrow under these circumstances.
If you don't have an agent, you have no one to blame except yourself and you are in for a prolonged expensive eviction.