Hi there,

At first only one answer should be possible for a question, how can i realise that?

Currently, there is no setting to do that. Perhaps it would be best if you can submit a feature request regarding this issues by creating a new ticket on the forums and set it to "Feature Request" here. We will prioritize this feature during the development process for the next major version of EasyDiscuss if there are more people request on this.

Only a higher role, not everybody should answer.

You can disable 'Post New Replies' which is at Backend > EasyDiscuss > ACL > choose the User Group that you don't want them to post reply > Post.