McDemott sank 12-of-16 free throws for the Bluejays (23-4, 13-2 Big East), who went scoreless over final 2 minutes but held on when Seton Hall's Fuquan Edwin missed a desperation 3-point try at the buzzer.
Grant Gibbs added 16 points.
Creighton reeled off a fourth straight victory and moved a half-game ahead of Villanova for first place in the conference.
Edwin finished with 21 points the Pirates (14-13, 5-9). Gene Teague contributed 14 points and 11 rebounds.