I'd take out Convolute, Condemn, Negate, and Cancel for Path to Exile, Dissipate, Hinder, and Puppeteer.

+1

if you have a way to make the stolen creature indestructible then you can keep the creature and keep stealing more.

Avacyn, Angel of Hope

cards that come to mind are Darksteel Plate , Deathless Angel , and Spearbreaker Behemoth for the big creatures and if you can get to her ultimate Elspeth, Knight-Errant . the obvious one is Avacyn, Angel of Hope .

I am thinking maybe I will put in a darksteel plate instead of one of my equipments already there, and a deathless angel in place of one of my big guys if I can acquire one. Unfortunately, Avacyn and Elspeth are out of my price range, while the behemoth is the wrong color.

dur, my bad

Huh, this is the only other Merieke Ri Berit Commander/EDH deck I've seen. I'm glad someone else recognizes her potential. She's can be the ultimate trump card if given the chance.

The previous comments are correct, indestructibility is a good way to keep stealing creatures, if a mana-heavy one. However, she has a trick up her sleeve in the form of mass control. By making good use of checks and priority, you can manipulate the stack in a way that lets you take control of as many creatures as you desire at the same time. You'll need to load up your deck with more than a few "untap target creature" abilities, but the payoff is worth it. A comment I left on my deck explains how to do it.

I'd also consider finding more ways to protect her. She seems innocent when playing with people you don't play often, but once someone figures out why she's so potent, she'll be an immediate target, and boy is she fragile. I'd recommend using all the shroud/hexproof equipment and auras you can get your hands on, even if they'd be sub-par by the Casual format's standards. Blue has tons of them. Also, this deck shouldn't be without card:Champion's Helm for any reason.

Check out my spin on it: The Siren's Army. Keep in mind that I haven't worked on it in awhile, and I don't have access to nearly the card pool you seem to, but you might find some ideas of your own. Use her wisely, she's more powerful than most people realize.