No one here can help, advise, or even point you in the right direction when your post is one vague statement followed up with nothing else about your situation.
If you are their PoA then read the document to see what triggers the legal authority to make decisions on their behalf. Often it is 1 or 2 medical diagnoses of incapacity.
If no one is their PoA, and they refuse to assign one, then you may need to begin calling APS, whether or not you live with them. If you live with them and they have an angry tirade that includes threats, call 911 to report that they are threatening and possibly delusional. The EMTs may be able to take this person to the ER, which may be an opportunity to create a care plan or move to a facility.
Gaining control when there is no PoA would ultimately mean seeking guardianship through the court. This is an expensive process. Otherwise, the county can become this person's guardian. Then you won't have to control anything but you can still carry on a relationship with them (or not).
Who are you caring for and what is their age? Is Dementia involved? Do you or anyone hold DPOA for this person? Do you live with them, them with you or separately. We can give you better answers if we get this info.
If no one is their PoA, and they refuse to assign one, then you may need to begin calling APS, whether or not you live with them. If you live with them and they have an angry tirade that includes threats, call 911 to report that they are threatening and possibly delusional. The EMTs may be able to take this person to the ER, which may be an opportunity to create a care plan or move to a facility.
Gaining control when there is no PoA would ultimately mean seeking guardianship through the court. This is an expensive process. Otherwise, the county can become this person's guardian. Then you won't have to control anything but you can still carry on a relationship with them (or not).