Thomas:
I'm not 100% certain I know what you're asking, but if you mean can you restrict the amount of time a resource enters into the Tasks page or Timesheet page to the number of hours that were planned for the task, then the answer is no, not out of the box. However, you could create a custom event handler that would reject submissions based on this criteria. Keep in mind that in the real world, people do work more than the planned amount, and ususally it's more valuable to know that, than reject it.
Gary Chefetz, MCITP, MCP, MVP
msProjectExperts
Project and Project ServerFAQs
Project Server Help BLOG