Hi aruba.markus,
There are a couple of things that you can do to limit bandwidth. Here's some examples:
1. Different user roles can each receive a bandwidth contract. For instance, Staff may get 25Mb and Guest may get 5Mb each. Clearpass, or another RADIUS server, can really help to pass different user roles to pull this off. You could even have different bandwidth contracts if you're in different OUs within an AD domain.
2. You can apply a bandwidth contract to a VLAN. All of the clients that you place in that VLAN will be under the umbrella of that bandwidth contract.
3. You can apply a bandwidth contract to an AP Group. This is really helpful in a branch office configuraton. Let's imagine that you have a 20Mb circuit. You can limit the wireless bandwidth to 15Mb total for the AP Group and still guarantee 5Mb total for wired services.
Hope this helps!
-Mike