Ralpez

Here you go bud:
Open up your main steam window,
Go to the top and click on "Steam"
Scroll down to "settings" and click on it
Find the Following tab: "IN-GAME"
You will see a small box saying: Screenshot Shorcut Keys:
Click on the X on the box to clear it out, Leave it blank,
Click OK, and Close out of the window.
Gratz!, You've just disabled steam Screenshot,
Hope this helped you out, JAM.

PS: If you WANT to keep the screenshot feature, You can remap it to a difrent key the same way as stated above, You can also remap the back up keys in your .ini if your using Rc2.