Buy a new shirt <smirk>
Once the stain has set it's pretty much there for life, though you could use bleach, if the fabric isn't white you're pretty much stuck with covering it up.
Did you google? I think I heard hair spray way back.
OH, I have a chemical in the garge that I was told would work on cloth stains... Off to look in the garage for the name.
Back. It is called Naphtha (from a hardware store). I was told by a shop that does VERY HIGH END boats that will remove about anything, but dap it and use care..
If it does not work. New shirt or a name tag to put over it.
Yeah, I asked my wife. She says hair spray works. She recommends washing it as soon as possible after spraying it, mostly because that's the way she does it.
I did a Google search, many hits. Several suggested hair spray here is an example:-
To remove ball point pen ink; spray liberally with hair spray. The ink will dissolve on contact. Rinse with cool water and repeat. Test for colourfastness of fabric first to make sure it wont damage the fabric.
My wife has gone off to try it. I'll post the results.