Mental competency

Is there a way to render my mentally disabled brother's signature NULL and VOID? He continually signs legal contracts (for the purchase of cars and other items on credit) that he cannot pay for. He is incapable of making rational/sound business decisions due to his 3rd grade mental capacity.

Anyone accepting your brother's signature should be quite aware from meeting with him, based on your description, that he does not have the ability to sign a contract. It is highly unlikely a court could ever be convinced that the people providing him the contract were not aware of his limitations. So if the contracts should not be upheld, legally you should not have an issue. However, even if that is correct, that doesn't solve the headaches of your having to deal with these issues. That practical issue is not simple to address. While you could advise people you think he might meet to be cautious, that's not simple or practical either.

You did not indicate whether you were appointed the legal guardian for your brother. You should explore this with a local attorney that handles guardianships so that you will have the legal authority to act on his behalf. You should also inquire from that attorney whether there are any other steps you can take to limit the problems of your brother signing contracts. If the guardianship appointment expressly clarifies the issue you might be able to use those documents to more quickly resolve any future "contracts" your brother signs. While that might help, you're still left with the practical issue to address.