The equity is vested, you own it. You have no obligation to return or surrender it or even sell it. This is why companies have vesting schedules. Additionally, starting a new company with the same idea with the same team would most likely open them up to legal liability. The only thing to remember is that the equity is only valuable if the company succeeds, so it may be worth compromising by having them buy you out, or giving the company an option to buy you out at the next funding round.

Do yourself a favor and talk to a startup lawyer (one that's not used by any members of your team). S/he will be able to explore your options with you and help you decide on a good course of action.