What you said
Try to track the causes. Has one child had a "problem" that has caused others to react? Has there been a general disruption to their routine? It never hurts to check this out. Check the lesson plans and resources with care. If they are not engaged with the units you're running, then it's going to be demonstrated elsewhere. Bribery and corruption also works, so think about stickers for those who behave. It's a golden oldie.
The expert view
Telling tales is usually stopped by doing the following. Every time someone tells a tale, tall or otherwise, nod and say: "Hmm. I need you to stay behind and write out in detail exactly what happened."
Then, if they are old enough, do not let them take less than 10 minutes to do so (tell them that you "need more detail"). If they are not at the writing stage, simply take the time to interview them at length, and make them wait a bit after lesson and during lunch until you do. It sounds harsh, but this discourages a culture where children use you as a weapon against their peers.
If any of the allegations are more serious than a nuisance, make sure you give them some of your real time immediately, in case you deter genuine problems from being aired.
But if any of the children tell a tale, and you can easily show that it is an invention, you need to show them some consequences: call home, make them miss playtime, and so on.
As for the slapping, reboot your behaviour expectations with a good stern talk to the whole class. Make it clear what will happen if anyone slaps someone else in your room. If you catch anyone doing it after that, they need a sanction that is immediate - so the same day - and stiff. Plus, make it very public that this is what you are doing.
Children will do what you let them get away with in most cases, so set a standard that this shall not pass.
Tom Bennett is author of The Behaviour Guru and Not Quite a Teacher. Post your questions on: www.tes.co.ukbehaviour.