In the short run, turn the water off at the wall.

Longer term, things to try (I am not a plumber, just the basics since your question is not very specific)--make sure the flapper is seating all the way down on the bottom of the toilet tank (open tank, push on flapper, or re-seat it).

The last several days I've been reading numerous messages about black stains in a toilet bowl. Many have stated it was in the water. If this was true you would have the same black stain where you use water. The simple fact is a problem is within the toilet tank itself. Instead of trying to constantly clean the bowl simply get rid of the problem. Test this yourself.
Take the lid off the tank and set it aside. If you have a separate shot off valve turn the water off. If not, simply hold the float in the upright position. This will also stop the water from refilling the tank. Then flushed a tank. At the bottom of the tank in the center you will see a black gasket. Simply take the finger and robe the outside of the gasket. Look at your finger and if it is black you upon the problem. Simply remove and replace a gasket sometimes referred to as a spud gasket. Try to locate a replacement that is not black rubber. Now you will have solved the problem.

from your topic line my question was gonna be 'as opposed to what other kind?'

those flappers wear out, they're cheap and easy to replace. go buy a new one and put in.

you can test it by putting some food coloring in the water in the tank part, DON'T FLUSH for a while and see if the color seeps into the bowl. don't use yellow food coloring. use blue or red.