We respond to them by saying:
1- Verse 3:169 teaches us that he is alive (spiritually) since he died in Allah’s way. And he has sustenance.
2- When we say Ya Ali Madad we are basically asking Imam Ali (a) to use his sustenance to help us, just like you ask someone to help you in this world.
3- Imam Ali (a) has been given the power of Shafa’a. It’s shirk if we worship Imam Ali, but we don’t. We are asking Allah, but we know Allah has given him Shafa’a, so when we say Ya Ali Madad we are requesting that Shafa’a.