Ingnoring them, why not, but contradicting them, no they can't. Or else it would be some sort of reboot . And the title EPISODE VII is there to tell us that it's really a sequel trilogy. (well some could say that the PT and the OT already contradict themselves from certain Lucas points of view... but well)
I don't see Lucasfilm completely turn their back on PT fans. I mean, love it or hate it, but the people who love the PT and Clone Wars are a huge part of the audience who kept Star Wars a succesful franchise. Even if I'm an old fan of the OT, I would be a bit sorry for the PT fans if the new movies just completely relate on OT nostalgia without assuming a bit of the PT.
There's only two ways to please everyone (and for 4 billions, I think Disney want to please everyone!):
- Make a sequel trilogy with elements of the OT and the PT (as well as new ideas that please everyone...!)
- Make a sequel trilogy without any element of the OT nor the PT. Make something completely new in the same universe that is so awesome that everyone would be pleased. (but in this case why keeping the word EPISODE in the title? Make it a "Standalone trilogy".)
That's why I think that if (as it seems to be) the OT characters are returning in the sequel trilogy, then some elements of the PT will be there too.
"Pleasing everyone" is not always a good way to make interesting art, but in this case I think it's the option that will be chosen, for Episode VII at least.
I could be wrong though... if they think their new ideas are strong enough, they could just use the OT characters to a minimum and almost make no references to the PT. Time will tell.