plant height

is it possible to keep a cannabis plant under 15inches?

Look in the indoor section.
People do it all the time.

how can I keep it this small?

READ, lots of stealth grows in there where people do a grow in a cup inside a PC box.
Since it appears you are a newb, and asking this question, I can only guess you are trying to hide it from Mom and Dad?
Sorry, but if that's the case, I'm not going to help further.

No, I've grown multiple times. just saw a couple threads and wondered if it was possible for such a small plant.

Dude started growing this tree in 1948.

If want to grow small study Bonsai. All these micro grows come from Bonsai it's nothing new.

Most indica strains will stay within that height (and some sativas too) if you go 12/12 straight from seed.

-spek