So I just got a call from these people saying my name and number was randomly selected to receive a grant of $8400. Last time I checked you didn't have to pay a dime to receive a grant. They must think I'm dumb lol.

They called me ask for a debit card number where to send this money to so I stop the card

I just received this same phone call as you Mizzman - Same "David Johnson" to call to confirm my grant.  As soon as I told him I didn't want it, he hung up immediately.  Knew it was too good to be true.  What's weird is that they had my home address and called me at work.  Creepy

I just received the same call and was told to call a one David Johnson being naive I wish i would of read the blogs first.I kinda knew it was to good to be true however i gave them my card number should cancel it even though there's no funds on it?

"there's no funds on it" suggests it's a debit or prepaid card.
If a debit card, you MUST tell your bank what happened and get it replaced because it's connected to your checking account.
If a prepaid card, you can let it go or throw the card away. If you add money to it in the future, you run some risk the scammer can empty it before you can spend it.