This setting is toggled on the Build tab of the project's properties page:
If there are warnings that you don't want to break your build, you can suppress them specifically. You'll need the warning number, which will be reported in the Output window:
This number, without the CS prefix, needs to be entered in the "Suppress warnings" box, on the Build tab. Multiple warnings can be suppressed, by separating multiple warning codes with a comma.
Alternatively, you can use a #pragma compiler directive in the code to suppress your warning: