Continuing on the subject of לאו שאין בו מעשה, here are a few more examples.
We are commanded not to test Hashem. The Torah says, לא תנסו את ה׳ אלוקיכם. We are not allowed to expect direct payment from G-d for acts of kindness that we perform. This is a לאו, a negative commandment, without an action.
The Pasuk לא תגורו מפני איש, not to be afraid of any man, is also a לאו שאין בו מעשה. We are to believe that no damage will come to us when we act justly. This refers more to a judge that he should trust that when he acts without favoritism, he will not be harmed.
A king is commanded, לבלתי רום לבבו מאחיו, that he should not act in a haughty manner over his brethren. This is Mitzva not only for a king but for all Jews not to allow ourselves to be בעלי גאוה, men of pride. We are to work on our humility and treat every person with respect and dignity.
These were all examples of the principle of לאו שאין בו מעשה.