"Mummy, sorry but I'm gonna bite you soon!" Hehe so cute!
Just kidding, but you'd have to be firm and consistent in your reactions - a simple thing you can remember is Pain, Explain, Ignore.
1) Letting her know that it hurts (because usually it does!). Don't scold her or scare her with your reaction, but a simple "Ouch!", "It hurts!", "Pain!" or whatever she is used to in expressing pain will do.
2) Explain to her that doing that hurts, and it's not right to be biting anyone no matter her reasons. If she's frustrated, then talk to her and try to understand why she is upset (even if she can't talk, reference to her body language and try to suggest reasons why she's upset so you can find the root cause) before addressing it! If she's just being playful, you'd have to take step 3 - ignore.
3) When she does this because she's playful, one of the most common reasons would be because she thinks your reaction is amusing. The simplest way to resolve it is just to ignore it - if you don't react or respond to her, she'd start thinking that it's no longer fun and would just stop doing it :)