Here is what worked for us and I still battle this today. It is tricky, once you start it is near impossible to go backwards and I wish I could go WAY back! We did sleepovers with only certain families in elementary and then that became challenging to explain to your child why you can at this house but, not that house? And who knows what they said to their friends. It's a terrible battle that little minds just can't understand and just as challenging with older kids too - although it's a tad easier to reason with an older child...sometimes! We pushed to more late-overs in middle school and then just left it at that for a long time - although there were some sleepovers thrown in there however, very limited.
Now that my kids are older teens in High School - we still do not prefer sleepovers and say no often - staying busy helps as we all know sleep is important. And I actually feel different about them for our daughter vs. our son which is extra challenging. All this to say - kids just love them so it's so hard to say no, so I wish I would have just said no from the very beginning as just a standard rule.