It says so in the Quran.
33:40
Muhammad is not the father of any of your men, but (he is) the Messenger of Allah, and the Seal of the Prophets: and Allah has full knowledge of all things
Regardless of that, if Ahmediyya community wants to believe Seal means "Certified" then they are welcome to do so.
The Quran also says:
109:6
To you be your religion, and to me my religion
Let Ahmadis argue their point of view, I'll argue mine. There is no need to kill Ahmedis since I think with the simple knowledge of Arabic grammar I can make a case against their position. But unless Ahmedis come in my face with that assertion, I do not even need to do any such thing as well. We should live and die for Pakistan, be together in harmony...
The only way I would need to kill Ahmedis for stating their point of view is when I don't believe in Khatamun Nabiyyun myself and go like "Oh crap, these guys are about to shatter my world view I must silence them". If you believe in the finality of the Prophet then so be it, good for you, each time somebody says so different point them to this Ayat. Case closed.