Question

What kind of argument or pact could King Hrothgar have made to stop Grendel's vicious attacks?

Answer 1

He could move his Hall Heorot so Grendel would quit assaulting them, in light of the fact that all things considered, Grendel just needs his domain all to himself. 
Unmistakably Grendel is a noteworthy reason for Hrothgar's affliction—there are just such a variety of times you can tidy up the blood and modify your mead-hall before you feel somewhat frail and broken.