Ah, I see. I don't think there's a way to force people into specifying a source or sourcetype, but you can force lower roles to specify indexes by removing their searched-by-default indexes. Keep in mind though, they can just specify index=* and you're back to square one.
Another way to reduce inadvertent load is to reduce the time range searchable by lower roles to a week, a month, or whatever makes sense in your roles' scenarios. Another caveat here, make sure you're not killing the usability of Splunk for them by restricting the range too much.
Consider giving them smart accelerated data models so they use the Pivot interface instead of building reports manually.
Depending on the scenario, build custom forms for certain tasks that hide the search language a bit.
However, the best thing to do is user education. Your server load will drop, and users' productivity will go up - win win.
