Once Expedition Lookout has legally attacked, causing its last ability to not apply by removing cards from graveyards won’t cause it to stop attacking. However, if you do so before blockers are declared (and it’s attacking in your direction), you will be able to block it.

Expedition Lookout’s last ability will apply if any opponent has eight or more cards in their graveyard, not necessarily all of them or even the one Expedition Lookout is attacking.