Can I stop my corporate client's employees from booking flights and hotels that break their company's travel rules?

Yes, in most cases you can, as long as the client gives you their travel rules in writing and those rules are specific enough for the system to check. Things like cabin class, how many days in advance a trip must be booked, which airlines are approved, hotel star rating and a ceiling on the room rate for a particular city can all be set as policy on a corporate login, so the employee simply does not see the options that are not allowed, or sees them marked as outside policy.

There are usually two ways to handle a booking that breaks a rule. You can block it completely, so the employee cannot continue, or you can let it through but mark it, ask the employee to type a reason and send it to their manager for a decision. Most companies prefer the second option, because there are always genuine exceptions such as a last minute client meeting or a city where nothing approved is available. Rules can also differ by employee grade or department, so senior staff are allowed more than junior staff.

The honest limit is that policy only works on what is booked inside your portal. If an employee books straight on an airline website or a consumer travel site, you have no way to see it or control it, so the client has to insist that all travel comes through you. Some rules are also only as good as the information the supplier sends back. If an API does not clearly return the fare type, the room category or whether breakfast is included, the system cannot reliably check a rule built on that detail, and what you can configure may depend on your plan.

Ask each corporate client for their written policy, pick the three or four rules that actually matter to them, and start with warnings and approvals rather than hard blocks so you can see how often exceptions really happen.