As you may know, “desu (です）” can be added to make a Japanese sentence sound polite. So in this case as well, “Muri desu （むりです）” sounds better. But it can be used without “desu” if you want to say it casually. “Muri” is used when you think it is impossible to do something. For example, a friend of yours asks you to lend him one million dollars, you’d probably reply, “Muri!” Or if your boss asks you to finish some difficult and time-consuming task by tomorrow, you’d probably want to say, “Muri desu” instead of just “Muri!” “Muri” always shows a refusal, but if you say to your coworker, “Muri shinaide （むりしないで）,” it means “Don’t work so hard.” It is a good expression since it shows your companionship with each other. Use it when you see your coworker always busy working.