In ICC for Windows, click the "Settings" icon at the top of the screen.
In the window that pops up, click "Formula" (1).
Then choose the operator (less than, greater than, etc.) (2).
Now set the rating range (3).
Remember to CHECK "Use formula in seek" (4).
In this case, I set it so that only players whose rating is greater than or equal to my rating+100 can challenge me.
To give an example, if my rating is 1600, only players whose rating is at least 1700 can challenge me.
Click OK and you're set!
Please note that this only applies to people who are responding to your seek, or challenging you. If you challenge someone directly, the formula won't apply. Also, your formula will not apply in pool games, such as 5-minute, 3-minute, 1-minute, etc.