Por mí, arráncatela y engrápatela en la frente is an extremely gory expression. Its literal meaning is "as far as I am concerned, you might as well gouge it and staple it to your forehead." The unmentioned thing that can be gouged and stapled is the other person's penis. It is a colorful equivalent to the expression "this is a free country" or "do as you please, I do not give a damn."
An old school alternative that evokes a less bloody image is por mí puedes hacer de tu culo un papalote. Its literal meaning is "as far as I am concerned you can make a kite of your ass."
In both cases, make sure that the person you are talking to doesn't get easily offended, because these expressions are utterly informal.
Profesor Jirafales: Con todo respeto espero que no te moleste que invite a tu mamá al cine.
Kiko: Por mí, arránquesela y engrápesela en la frente, profesor.
Mr. Jirafales: With all due respect I hope you don't mind if I ask your mom out to catch a movie.
Kiko: As far as I am concerned, Mr. Jirafales, you can gouge it and staple it to your forehead.