minute 00 through 59
hour 00 through 23 (midnight is 00)
day-of-month 01 through 31
month-of-year 01 through 12
day-of-week 01 through 07 (Monday is 01, Sunday is 07)
All five fields must be specified, but an asterisk can be used to match any time/date field. So to build an entry that would run "command" every fifteen minutes you would create a file with a text editor and place an entry in it like this:
15,30,45,59 * * * * command
Which would run "command" at 15 min, 30 min, 45 min, and 59 min, of every hour, of every day, of every month, of every week.
Please be as specific as possible in the future as your question was properly addressed in my previous response. You will have to most likely write a custom script (or have one written for you). That will check all the cron tabs once every night to make sure that no one has created one. I don't know that you can exclusively limit them unless you can totally disable them from doing cron jobs at all and make them e-mail you each time they want to add one.