Blue, as you might expect, is a bit sneakier. To begin with, there is a rather healthy amount of bounce available. Just keep in mind that Wash Out, Hibernation and other color-based effects probably won't work as well. Once you get to five or more mana, more options become available. Once you get to eight mana, you can skip searching your opponent's library and just take the one they already have, via Dominate.
Nothing beats force-feeding an evil mage a bit of their own medicine.
The one that would be good enough to see some play is Tsabo's Decree, which is already being used in a lot of sideboards and even some main decks anyway. Gold Most of the gold cards are variations on themes that they take from their various colors. Only a few of them are viable. Terminate, of course, is one of the most obvious anti-Spiritmonger measures, and has the advantage of whacking just about anything else as well. If you are already playing one of the two colors, you are probably best off just splashing the third and packing four of these.