YB: YES. A Lo Sa’ase is aways Doche an Asse.
For example, Shabbos, If one has to work on Shabbos for food for that day(otherwise he is fasting) it is assur to work. Because working is Lo Saase and Fasting is the absence of eating which is Asseh.
About your impure thoughts you might want to be more careful on what you look at. You obviously have access to the internet – What are you seeing?
Also practice during the day to get rid of those thoughts. If you let it harbor when you are not Davening you have no control when you are.