It would seem you started 2 threads about that topic, and I answered to the other one, wondering why you'd think a peace sign devilish at all.
It is certainly okay for you to voice your opinion, but it's also okay for your gf to disagree. And she has a point in what she says. The more we are scared of Satan the more power we give him over our lives. There are some things that are clearly evil and destructive and we have to flee from them. But seeing Satan behind a merely saecular (and rather positive) symbol like the peace sign.
A symbol is a pattern of lines and colours. It has no spiritual power in itself, all power a symbol has comes from what you attribute to it. So the goodness or evilness of a symbol is not within the symbol, but within your mind.
So if your gf sees a peace symbol as just that, a symbol of the wish for peace among people, then that's exactly what it is for her. Nothing satanic attached to it. But if you want her to avoid peace symbols because they are satanic for you, you ask her to give something to the Satan that wasn't Satan's in the first place. You'd basically ask her to make a sacrifice to Satan out of fear.
Geez I hope this makes sense.