My girlfriend's best friend is hitting on me, what should I do?

I have been with my girlfriend for 6 months and everything is going great between us. But for the last couple of weeks, her best friend has hit on me when we hang out altogether. In particular, she rubs her foot on mine under the table.

I ignored it until now thinking she'd understand I am not interested, but she has done it on three occasions now and I'm starting to feel very uncomfortable and don't know what to do. Is the best thing to just keep ignoring it? Do some people on here had that happened to them, what did you do, how did it end?
My girlfriend's best friend is hitting on me, what should I do?
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