Malaysian cast their vote on 9th May 2018. The procedure was setup that whoever casting their vote will be marked with ink on index finger.
Myself got into casting the vote and had my index finger inked by the officer taking care at the voting centre.
I have left the index finger with the ink soaked in chlorox mixture with water for 10 mins 
You can also try out using paint thinner for scrubbing it off.
Another person said to try out by soaking in soya bean drinks for 20mins and scrub it off. Not sure if it works for you. Good luck in trying.