How can I prevent water from running off a patio to my foundation?
Sealing that joint is a low-percentage game. The patio probably moves with seasonal changes in ground temperature, and most products will let go after a few years, leaving you in the same position. 

I'd do this:


Remove some decking to make access to the patio. 
Using a rented diamond saw (chain saw style) or jackhammer, cut 2-3' off the foundation side of the old patio. 
Re-grade with suitable soil and slope between the remaining patio and the foundation. The new slope probably wouldn't need to land above the patio if there's a suitable side outlet path. 
Reassemble the deck and be happy.