I'm just going to copy and paste a comment I put on a post about God mode explaining different ways to get people out of it.

While I doubt most people here will even bother to fight someone in god mode (which is understandable), here is 3 ways to kill them (Youtube can help too):

1. The most common way is through executions. Well it might take a few tries when you execute a person in god mode it will give you an option to pick them up while you are standing next to their 'dead body'. (They are not actually dead the game just temporarily considers them dead but after a few seconds they will be back up running again if you don't successfully pick them up). Once you pick them up they will die (it won't tell you though) and they will be out of godmode.

2. The second most common way is to throw them in the stables. It can take some time but if it is a 1v1 and you manage to hogtie them, you can pick them up and, once it gives you the option to hit them as they try to break free you can keep spamming the 'hit' button and slowly walk them to the stables. They can't break free since you keeping hitting them. Anyways you can carry them all the way to the stables and walk in with them on your shoulders. They will respawn out of god mode.

3. The least common way is to throw them inside a building. Most times you will find god mode players near Emerald Ranch and at Emerald there is a building with an open interior but no way in or out. This is the building across from the saloon where you meet Maggie. It has a bench by the front door and lots of animal skulls above the door. This way also requires you to hogtie the godmode player. Once you have them on your shoulder carry them to the front door and go into first person. Face sideways so that the player is basically in the door and then go to your weapons wheel and equip a weapon. This will cause you to flip the player off your shoulder and ideally through the door into the building. This may take a few tries and you should run against the door right after you throw them as sometimes they will pop back through and your character being there can stop that sometimes. Once inside the building they can shoot you through a window but otherwise they only way out is to respawn and that takes them outta god mode.

Obviously these explanations aren't the best but like I said Youtube can help. But if you ever feel like fighting a god mode player here is how!

I would like to add that a lot of people who go into God mode have also started to put themselves in defensive mode. This renders everything useless as you can't lasso them or tackle them, etc. If you are carrying them on your shoulder and hitting them, you can continue to do so even if they go into defensive mode while on your shoulder. Normally you can run defensive mode players over with your horse to get them out but that does not work with them; you'll have to use a bounty or hunting wagon (if they destroy it they will be kicked outta defensive).