What I found to be effective AND hilarious is to say "hey, I get it. You're curious and turned on at the thought of having sex with a trans woman of color. It's okay." then walk them through some steps about jerking it to trans porn until they're comfortable with what turns them on, and then to make contact with the local trans community. Encourage them to find a real trans woman of color who be willing to have sex with them.
That usually gets them to stop posting about it.
Caption this Meme 