What is the ruling on someone who says to another: "Either pray properly or do not pray at all, for example, he sometimes prays and sometimes abandons prayer, and there is no benefit in your prayer which does not deter from indecency and evil," and does the one who says this commit an act of disbelief?

The belief that prayer is useless is disbelief (kufr). However, the phrase might be uttered with the intention of encouraging the preservation of prayer and explaining the consequences of abandoning it, not with the intention of negating its benefit. It appears the questioner is suffering from obsessive thoughts regarding matters of disbelief and others. He is advised to fear Allah and not to give in to these obsessions.