How can I grey out/disable certain fields?
If you're using Webform 3.x, you're probably looking for the Webform Conditional module.  If you're using 4.x, conditionals support is built in.

Adding conditionals to your webform will allow you to show or hide webform fields based on answers to other fields.  Using the above module or built-in support in 4.x, you could completely hide a certain field if the answer to another field was "Yes."