Most Helpful Guy
Depending on how close your relationship was with that person, but in time you'll forget. it also depends on your mental strength. If you want to forget someone, initially what you need to do is to get away from that environment, go to another city, relocate or at least go out on a vacation and importantly don't ever contact that person again, even if the other person wants to contact you.
Most Helpful Girl
If the person had significant impact on your life... it's going to take time to forget this person.
I would start by deleting their phone number, and them from social media.
I would also start by not contacting them or answering their calls.
I would also start by getting rid of anything they have gifted to me.
I would also start by allowing myself to heal and knowing that time is the biggest healer.
You don't really ever forget any one.
You get over them though.