[nightpool]

Originally Posted by joelg236

This exactly. Just add something similar

"This code and all associated files cannot be used in any commercial software, regardless of purpose."

Feel free to be as "legally-savvy" as you'd like. You could even copy that kind of clause from another non-commercial license.

Please don't. These licenses are constructed the way they are for a reason, and made by lawyers who know what they're doing. Adding a clause that restricts the usage of the software goes directly against the FSF's Four Fundamental Freedoms:

• The freedom to run the program, for any purpose (freedom 0).
• The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
• The freedom to redistribute copies so you can help your neighbor (freedom 2).
• The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.

Additionally, there are a lot of loopholes in a license like what was recommended, specifically that it is self-contradictory, as the GPL explicitly permits commercial use, and that you never define what constitutes commercial use, so its ambiguous.
For example, one of the terms of the CC-BY-NC license that covers commercial use:

You may not exercise any of the rights granted to You in Section 3 above in any manner that is primarily intended for or directed toward commercial advantage or private monetary compensation. The exchange of the Work for other copyrighted works by means of digital file-sharing or otherwise shall not be considered to be intended for or directed toward commercial advantage or private monetary compensation, provided there is no payment of any monetary compensation in connection with the exchange of copyrighted works.

As you can see, its a little more complicated then "you can't use it for commercial software"