How do we respond to those who associate partners with Allah in worship, who justify their visits and sacrifices to saints by claiming these saints are "intercessors," even though they know that Allah is the Doer of all things?

Visiting graves is permissible and encouraged for all Muslims, unless the visit involves a prohibited act, such as the grave being inside a mosque, or acts of polytheism like slaughtering or making vows being practiced there. In such cases, the prohibition is due to the denunciation of evil and a warning against it.

As for slaughtering for the deceased, making vows to them, supplicating to them, or seeking their aid, all of this is forbidden and a form of polytheism (shirk) with Allah; because it is directing worship to other than Allah. Their claim that they do not believe the deceased can bring benefit or harm, but rather they do these things to seek closeness (to Allah) and attain intercession, is of the same kind as the argument of the polytheists: (WE ONLY WORSHIP THEM THAT THEY MAY BRING US NEARER TO ALLAH) and (THESE ARE OUR INTERCESSORS WITH ALLAH). Allah has commanded that supplication be directed to Him alone, saying: (AND THAT THE MOSQUES ARE FOR ALLAH, SO DO NOT INVOKE WITH ALLAH ANYONE), and He commanded that sacrifices be made for Him alone: (SAY, "INDEED, MY PRAYER, MY SACRIFICES, MY LIVING AND MY DYING ARE FOR ALLAH, LORD OF THE WORLDS"), and the Messenger of Allah, peace be upon him, cursed whoever sacrifices for other than Allah.

If these people claim that slaughtering, making vows, and supplication are acts of worship, then they should be told that directing worship to other than Allah is polytheism. And they should be told: It is not authentically reported from any of the companions of the Prophet, peace be upon him, that they supplicated to a grave, or sought its aid, or sacrificed for it, or made vows to it. So, what was not part of the religion then will not be part of the religion now.