Given how its widely known as a poisoned gift, why do people keep accepting Harrenhal when its bequeathed to them by the Iron Throne? Is it possible to refuse ownership of Harrenhal?

They do it for the same reasons that people have over-reached themselves throughout IRL history: ambition and greed. 

Harrenhal is one of the largest, richest, and most important fiefdoms in all of Westeros, putting one at a level of being able to marry into the Great Houses. Unless you believe in the folktales, and it seems like most highborn people and smallfolk not from the area don’t, turning it down would be unthinkable.