Hi,
an Access Point is periodically sending beacon frames that conatain the ssid and so on...
So a client that is passive scanning will know about the AP.
Is it possible to suppress this and only answer to probe requests? (so active scanning)
You can hide the SSID, this causes beacon frames to have an empty SSID field, and the AP will not respond to probe requests unless the correct SSID is included in the request.
Beacon frames are important for establishing and maintaining links to authorized stations, so they can't be eliminated entirely.
Interesting. 
But I want something different... 
Hiding the SSID means that the network will completly be hidden from the client if the client does not know about the network...
I'm already supressing the probe responses... But I want to supress the periodically sending of beacong frames... ^^
A beacon frame contains the SSID. See: https://en.wikipedia.org/wiki/Beacon_frame
With that in mind...yes, as @mk24 stated:
Most importantly:
You then stated:
and
By hiding the SSID, you suppress:
Therefore, in order to suppress beacon frames in the manner your describe, you must hide the SSID.
Wow! ...so how do your clients see the SSID on their screens?! You stated above that you only wanted the AP to answer to probes!!!
...If you don't see your solution in @mk24's answer, you may wish to read more technical documentation on the makeup of 802.11 frames.
Totally forgot about this thread.
Actually, I wanted to steer clients by suppressing probe-responses from APs. Some clients did not care and just connected to already known APs.
Hmmm. Maybe it would be interesting to have some mechanism to hide and not hide the SSID dynamically. 