[angersock]

Is the contract "at-will"? If so, you can just quit before you start.

If you want to be a little nicer about it, say that you have a competing offer and if they don't match walk.

You don't owe them anything.

[throway_bayarea]

Thanks for the advice, the contract is "at-will".

Even if they match the offer I still prefer company B :/

[angersock]

Well, quit with apologies. "Due to circumstances outside my control, I'm unable to join at this time. I'm very sorry."

The sooner you get back to them the sooner they can find one of their other candidates.