I feel for you! I would think you need to be firm, really firm. Have them play inside, not outside (easier to direct). If they break a rule (that you have clearly laid out) then firmly, clearly and explicitly tell them it is time for them to go home, they can come play another day when they are ready to listen and behave. Every time they come, kick them out when they don't follow your rules.

It sucks, nothing like having to be disciplining other people's kids. But, if they are not being supervised and are inviting themselves over then you need to either just send them home without letting them in, or let them in and send them home when they aren't behaving. At 6 and ( years they are old enough to eventually figure out how to act around you. If not...then they will eventually chose to not come over, as they will get sick of being kicked out!

Their parents may not like it...but ultimately if they are not supervising then you are in charge. Your house, your rules.