How do you rule?

[Headhunter13]

We have a $2-5 NL home game. 8-10 players, we rotate the deal. On the flop, player A bet $50. Player B picked up a stack of chips, counted them out in his hand, and said, "well, here's your $50 and I'm going to raise another $100."

He's a relative newcomer to our game, but not to poker. As soon as he said it, everyone stopped and looked at me. I happened to be dealing, and the host of the game happened to be in the hand. Would you rule it a string-raise and disallow the raise, or let the raise stand?

Would it matter if he had done something similar earlier in the night and we had disallowed it then?

Thanks for the input

-- Headhunter13