Solutions are only shown after the answer date. So just set the answer date far enough in the future to when you are okay with students seeing solutions.

If you are viewing problems as an instructor the solutions will be shown to you even before the answer date. However, it will also say "Instructor solution preview: show the student solution after due date" (for versions of WeBWorK prior to 2.18) or "The solution shown is an instructor preview and will only be shown to students after the due date" (for 2.18, this is also below the problem).