How do we stop my mother with Alzheimer's from driving so recklessly?

The only way to take charge of this extremely hazardous situation is to TAKE CHARGE! If perchance your mom was to injure someone while driving recklessly, and your family was not only aware of the diagnosis but also of the danger, you would have to bear some responsibility for the results. Although taking away someone's independence by having them surrender driving privileges may be the most difficult decision you'll have to make, the possible catastrophe, that can easily occur if mom continues to drive, is far worse.
I strongly suggest that you find someone in a responsible position outside of your family to tell mom she can no longer drive. Chat with her primary care physician and/or neurologist; ask each to write a 'prescription' that says "Mrs. X can no longer drive". Discuss this with her insurance holder! Talk to the local police department and RMV for possible means to keep her from driving. If it is her automobile, get it out of sight and use the wonderful 'fiblet' "the car needs repair". This buys you a bit of time while finding the best way to keep her from being behind the wheel.
It sounds as if you may be waiting for the wreck to happen before something is done. Many family members don't want to be the one to incur a parent's anger by taking away another part of their control that is being dissipated by Alzheimer's (AD). Please accept that she may blame you for not being able to drive but having an upset parent is far better than seriously injuring an innocent person. If this does happen, remind yourself that she is not really angry with you - she is angry at the disease. AD folks frequently feel they remain good drivers because they are able to get to familiar places without becoming lost. However, safe driving also involves: * decision making * multi-tasking * hand-eye coordination * rapid response * good memory & attention span All of these areas and more are affected by the presence of AD. Chances are all of these metioned above are already affected by your mom's AD. As for your dilemma with the unsavory character who is stealing mom's finances, you do need to report this to the police department and her bank ASAP. If you, or some other family member, does not already have a Durable Power of Attorney and Conservatorship do see an Elder Law Attorney and get these most important instruments into play and again do it ASAP. These powerful documents will make it so much easier for you to rest assured that mom is being taken care of to the best of your power and that a plan is in effect to protect her from perpetrators and scams. You have a lot on your plate; please take care of YOU.