This pattern has two main uses. First, it conveys a strong recommendation not to do something, based on social or moral norms. It's like saying 'one should not' or 'it's not the kind of thing to do.' For example, 人を騙すものではない means 'You shouldn't deceive people.' This is often used in advice or admonishment.

Second, it can express that something is impossible or beyond one's ability, especially when used with potential verbs. For instance, そんなにたくさん食べられるものではない means 'It's impossible to eat that much.' In this case, it emphasizes that the action is not feasible.

The tone is somewhat formal and literary, more common in written language or formal speech. In casual conversation, you'd more likely hear ～べきではない (for should not) or ～わけがない (for impossible). But ものではない is still used in set expressions and written advice.