And even as they did not like to retain the Lord in their knowledge, the Lord gave them over to a reprobate mind, to do those things which are not convenient; Rom 1:28

The most telling characteristic of a person with Reprobate Mind is that they do not consider God in their knowledge or thinking.   

This leads them to say and do things that are "not convenient".   
"Not convenient" means "not normal", "not common-sensical", "not sane".   

Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Rom 1:29

Backbiters, haters of the Lord, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Rom 1:30

Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Rom 1:31

Who knowing the judgment of the Lord, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them. Rom 1:32

From the above we see that their disconnection from God results in all manner of sin.   The extent of their depravity is that they enjoy the fact that others participate in those sins.   

The reality is that the decision to deny God leads inexorably to a Reprobate Mind.   Disconnection from the knowledge of God has negative short-term, long-term and eternal consequences

Heb 11:6 says that a person cannot come to God without believing that He is .   Psm 14:1 says that a person who denies God`s existence is a fool.   

So, God does not have to do anything to a person for them to have a Reprobate Mind.   They do it to themselves.   It is an act of their will.   God respects their decision and gives them over to their desire.   

No one can make such a statement without having rejected the Truth.   

The Word is Truth Joh 17:17.   
Jesus is Truth Joh 14:6.   
Holy Spirit is the Spirit Of Truth Joh 16:13.   

Do not waste time with such.   Simply tell them that they will come to know the Truth when they bow before Him in judgment just before being cast into the Lake Of Fire Rev 20:15 .   

If a person asks you how you know the Bible is true they are seeking Truth.   Such are open to the Truth.   Witness to them of your experience in Christ.   Share with them the fulfilled prophecies - especially those regarding the Lord Jesus Christ.