You don't. You've opened the floodgates.
However, there's a hardware mechanism you can use if you've got a centralized N-Tier and these users aren't running the app off their PCs. Firewall your SQL Server and only allow particular IPs to access it, and then open up those IPs that the application/N-Tier resides on.
Never stop learning, even if it hurts. Ego bruises are practically mandatory as you learn unless you've never risked enough to make a mistake.
For better assistance in answering your questions[/url] | Forum Netiquette
For index/tuning help, follow these directions.[/url] |Tally Tables[/url]
Twitter: @AnyWayDBA