If you find your Tumblr hacked or posting the “Tumblr Staff Blog $50 Starbucks Card Giveaway”, here’s a general way to fix it…
…totally delete all the custom HTML code in the “Customize Your Blog” thing (but if you have special shit in there, like a music player, copy and paste it to save it or some shit, but make sure it’s totally empty), then re-download your theme. There, that should take out the coding that the hack uses to put in the “You must verify you’re 18 years or older thing” that prompts the user to imput their login information, therefore, it should no longer show up anymore. Taadaa.
But after you do this, make sure you change your password to something completely different than your original password. If it’s possible to change your e-mail on Tumblr, you should probably do that too.
See? Problem solved.