How to disable spotlight for Alfred
In the "System Preferences" app select "Spotlight" from the top row. Now, uncheck the box titled "Spotlight search keyboard shortcut:"

I set the Alfred keyboard shortcut to Command-Space, which works a bit like Alfred on stock Yosemite machines.

If you want to keep the ability to search Spotlight with a keyboard shortcut, you could also change the shortcut using the popup menu to the right of the checkbox.