Well, Fluid is Webkit and there's no way to install plugins so you are pretty much limited to Glimmerblocker in that regard.
Depending on the situation though I may use a userstyle that has ad blocking built in. For instance, if I use my Fluid app for Facebook one of the userstyles I have to make FB look the way I want it also has code to remove ads.
Without the ability to customize like the Mozilla browser you are pretty much limited.