When I crate a Webform I do have an option to make it available as a block as well. But in this case it creates a node as well. How can I create a Webform as a block only?
I'm not aware of a solution different from the "create a webform as node" (+ make available as block).
However, to avoid anybody would want to visit that "node of the webform" (cfr. your "I don't need people get on node of this Webform"), you migt want to consider a variation of the approach detailed in the answer to my question "How to specify a Rules event like "Content is 'going to be' viewed"?". All you'd need to change to that "rule" is:
- the actual path to be checked (matching the path of your node of the webform, instead of node/*).
- and the no_access path (replace it by an appropriate path in your site).