You can modify styiling using many different techniques.

I think several of these options are possible:
1. You can explicitly say that the grid should not use styling (setting the appropriate grid property)
2. You can explicitly say that the grid should use a different styleset (using another grid property)
3. You can modify the <infragistics.web> web.config key to set a different styleset for the entire app
4. You can also provide overriding css for a limited subset of the grid's visual representation.

But you should be asking yourself: Why do I not want to utilize a coherent styleset for my entire web app? In other words: I guess establishing a well-defined styleset using appstylist and then sticking with this choice by referencing the style from web.config (and thus using it for all UI elements throughout your app) is the proper approach.

But still, how do you get rid of the styles? How do you tell the grid to stop using a particular style or class?