Generally you aren't able to limit users to 30 minute increments only. You can limit minimum and maximum length or reservations. For example you can limit a user to minimum of 1 hour and max of 2 hours. But within that range they could reserved for 1 hr, 1:15, 1:30, 1:45, or 2 hrs. You can also use the Block rule type to create EXACT time blocks that users must reserve. For example 9:00 AM - 10:30 AM, 10:30 AM - 12:00 PM, etc.