This is the logical fallacy/argument that attempts to reduce many options to two, and then forces you to pick on, even though neither apply.
The most common one you will see when debating theists is “If you aren’t for God then you are for Satan.”. Obviously this is incorrect. You are for neither. But how can we get the theist to understand this?
Well, the simplest way to attempt this is to present it to them in a way that works against them. For example, if they are Christian and believe Allah to not be a god, then tell them that if they are not for Allah, they are for Satan (Muslims have a Satan too, although he is a little different to the Christian one. Still considered a bad guy though). You will undoubtedly get the response “No, I am for God, the true God” or “I am not for either of them.”.
If you get the latter, lucky you. You have just managed to get them to discredit their own claim.
If they tell you that you MUST be for (their) God or Satan, get them to full explain why. Ask them how other religions can exist, or how atheists can exist if the ONLY options are God or Satan? Most likely you will hear “Because there is ONLY God or Satan and you MUST pick a side.” This is a self reinforcing argument that many theists use in order to avoid having to face their false dichotomy. Simply proclaim that only their veiw of the world can be the right one and that you are part of it whether you believe it or not.
You could attempt several methods here. The fastest, although not exactly the politest is to set up your own false dichotomy. Tell them that they are either a murderer or a rapist. Which is it? That they MUST pick one. There is nothing else that they could be so which one is it?
While this may seem extreme is should also provide enough of a verbal slap to make them see how their original claim is wrong. Many theists will simply label you as a satanist or devil worshiper if you say anything other than that you are for god, and there are unfortunately not many ways around this.