Ctrl + Click -> Peek Definition is actually enabled by Productivity Power Tools. If you disable "Ctrl + Click Go To Definition" in Productivity Power Tools options then this behaviour will disappear.
If you have Resharper AND Productivity Power Tools, by default this will cause ctrl + click to go to definition AND peek the definition. Very annoying. The solution from @SnowFatal will turn off the peek functionality. Resharper's option is listed as "Go to Declaration on <Control + Left Click> in the editor".