To keep them out of Greenland, you need to stop their expansion east. Or prevent their very existence. Once they get to the Eastern Arctic, they're going to get to Greenland. Don't know what could slow their expansion.
No Inuit in Greenland means none in Labrador, for instance, and possibly none in Ungava (northern Québec). Doable, of course, with a PoD around 800 or so, but it's not just Greenland that would be affected.
If the Dorsets developed a package that allowed them to do the High Arctic, that might well butterfly away the Thule (proto Inuit) completely. And, in any case, if it's the Dorset who are in place in Ellesmere Island, then THEY would have reached Greenland, which technically meets the OP request, but not really.or you could keep the Dorset in Ellesmere Island/Northern Greenland. You'd need to give them some cultural/technological advantage which allows them to thrive in the High Arctic while the Inuit displace them farther South. Not sure what that would be, but I seem to recall that the Inuit entered Greenland via Ellesmere so keeping them out of Ellesmere could keep them out of Greenland.