For Inventor Fusion 2013: Find the Constraints folder in the browser and expand it. Right click on listed constraint, use Delete.
For Fusion 360: Find the Joints folder in the browser and expand it. Right click on listed Joint, use Delete.
This is pretty standard for anything that gets listed in the browser. By showing model items of all kinds in the browser, such as bodies, components, sketches, etc., Fusion gives you a convenient place to perform actions on these items, such as delete, copy, visibility, etc. In complex models it can be easier to use the browser for selections.
HP EliteBook 8560w Win 7
13" MacBook Pro, mid 2009 OS X 10.8.4
Feel free to ask more questions, we're always glad to help. Otherwise, please mark this with Accept as Solution so that other users can benefit from it.