A 'F2P' as an insult is someone in TF2 who sports the following three cosmetics:
Ghastly Gibus
Pyrovision Goggles
Mercenary Badge
and has very little knowledge and skill in the game.
I have translated the three, Gibus, Goggles, and Badge, into Touhou for a 'F2P' in Touhou.
Badge = Patchouli waifu with little actual knowledge of her
Goggles = Flandre and/or Koishi cutest thing ever with little actual knowledge of the two
Gibus = Word-for-word using classic Touhou memes with little actual knowledge of them
If you see anyone with the three mentioned above, you have just discovered a F2P of Touhou. Congratulations. Feel free to confuse their young, weak, and little minds with your immense knowledge on the subject.