My book HASHISH FROM QADIAN has all the answers to their lame arguments.
The Qur'an repeatedly mentions prophets/messengers before Muhammad (S) and none to come after him.
2:4 And those who choose to believe in what has been revealed to you (O Messenger) and the truth in what was revealed before your times. And they have conviction in the Law of Requital, and, therefore, in the Life Hereafter.
30:47 We did send Messengers before you to their respective people, and they brought them clear evidence of the truth. Then, We (Our Law of Requital) inflicted Our retribution on those who stole the fruit of others' labor and violated human rights. And We have made it incumbent upon Us to help the believers.
40:78 We sent Messengers before you. Some of them We have mentioned to you, and some of them We have not mentioned to you. And none of the Messengers was given the authority to bring a Message or a sign but by God’s leave. But when (the period of respite was over and) the command of God came, the matter was decided in truth and justice. Thus, right then, perished the upholders of falsehood.
And much more.