Is their any agency which would remove a person who is harmful to himself or others due to Dementia?

You need to call APS, Adult Protection Services, in your county.

If this person gets physically violent call 911. They can be removed and sent to a Psychiatric facility to get and manage meds for the violent tendency. After that, LTC will be needed.

Agency or facility? I believe that If a resident poses a safety risk to themselves, other residents or staff and themselves that facility will transfer that person to a geriatric psych ward for medication management in an effort to find proper medication to contain that behavior.
Facilities have a responsibility to protect everyone - residents and staff alike.