Company work hours by default don’t mean everyone has to work them. They only mean company business hours, when someone can come in. Small companies usually have the most flexibility in this regard, so it’s natural for people to assume that flexible hours is a norm for non-customer facing roles. Is this a helpdesk or sales position? Is it a software developer / network admin / another technical role? Two different treatment plans.

Regarding how to deal with this or similar situation (when employee requests a non-standard accommodation), you need to try your best to accommodate them. Talk to your boss, see what you can do. If you can allow some flex in work start/end time, do so. If after all this you are absolutely sure hours are strict, you must provide a reasonable explanation why this would be the case (unless it’s obvious to everyone, like a bank teller – need to cover a specific shift). In 2017 you cannot just reject them “because I said so”.

I’m serious. People are not going to change. IT is like any other industry, you get the good, the bad, the ugly and the strange.

You can control none of this. The only thing you can control is your reactions to what is going on around you. You don’t know why these people were hired, you won’t control whether they grow and expand or remain where they are or drift off or get fired.