This is a very common problem! People with dementia often become suspicious and accuse family and caregivers of stealing from them.
Try assessing the situation. For example, is it always a loved one’s purse that is missing? If so, you might try putting the purse in a place that this person can’t move it and forget they did so.
You can put a sign up that says something like, “Your purse is on top of the refrigerator” and then when your loved one thinks it is missing you can show this person the sign and gently guide them to the refrigerator and show them the purse is there.
It is fine to apologize and take the blame, then move on to another subject. Your loved one isn’t able to retain the facts anymore, that is why taking the blame and changing the subject can sometimes calm the situation quickly.
Simply letting him or her vent for a moment, taking the blame and changing the subject may be enough to get your loved one to calm down.