What doctor treats diseases of the rectum?

A family physician can diagnose hemmorhoids and give advice about treatment and how to avoid them, and may even prescribe an ointment to sooth them.

But, to remove hemorrhoids when they get too large, a surgeon is usually called in to do a hemorrhoidectomy. This used to be done by cutting and stitching, then it was done for quite a while by putting a tight rubber donut around the base of the hemorrhoid to cut off its blood supply so it will later drop off. This is called 'hemorrhoidal ligation'.

These days, most hemorrhoidectomies are done by freezing the hemorrhoids with liquid nitrogen to make them die and later fall off. This is the most pain-free method.
Your PCP should be able to help you, or refer you to a proctologist if necessary.