Debt collectors have the right to contact you, but you have debtor's rights, too. Debt collectors aren't allowed to call you nonstop, insult you or make you feel lousy for not paying. They also aren't allowed to call you before 8 a.m. or after 9 p.m. unless you somehow agree to it.
If you're being harassed by a debt collector, you can send a complaint to the Consumer Financial Protection Bureau or your state's attorney general.