I don't think you can keep the plant from blooming when it wants, but I've read you can store buds in the refrigerator and keep them several weeks. There are instructions in this link on how to store buds:

http://www.pendletons.com/peonies.html

I've never tried this myself, but I think I'll give it a whirl this year. Peony season is way too short!!

Here's another good description of the same process at this link -scroll down to "Bossing the Blooms"

http://annieinaustin.blogspot.com/2007/05/bossing-blooms.html

This lady says the blooms need to be out of the refrigerator 24-36 hours prior to when they're needed.

Martha Stewart had a great show on about this peony storage you are asking about, its on her web page.......how it was done seems easy....