Yes, it is possible to fight war crimes without leaving your residence, because we can all take a stand and make a difference. You can sign a petition online. You can give money to a worthy cause. You can post an encouraging message on the internet. These are all ways to stand up to war crimes.
It is my opinion that is indeed possible to fight war crimes without leaving the safety of one's own home. By donating to various charitable organizations it is possible to fund groups that are in opposition to war crimes, and therefore limit the possibility of such crimes occurring again in the future.
War crimes being carried out by a country can definitely be fought without actually going to war with that country. You can fund the resistance army like the United States has done in the past, the UN can impose sanctions, and individual countries can close their ports, or otherwise stop trade with the offending country.
Unless you're able to hold a trial in your front yard, fighting war crimes without leaving your house is going to be very difficult. Contrary to the common, and very earnest, belief of many people, you just can't get justice through the Internet. I guess you could feel good by donating money to some group helping with war relief.
I believe most people have very little power when trying to fight war crimes. I have my doubts that most people could fight war crimes without leaving their home. As an American citizen I can't change the fact or do anything to fight the war crimes my country committed during the Iraq War, rather I leave my house or not.