Should we say, A hassles with B or A hassles to B?
What is the right expression?
Neither. Hassle can be a transitive verb, so you can just say A hassles B.
A hassles with B is grammatical:
(though this example arguably uses 'hassling' adjectivally;
in 'the customers were hassling with merchants over high prices', hassling is indisputably verbal.)
But I'd guess the transitive usage is more common (but it does have a different shade of meaning).