Currently it doesn’t seems possible to exclude the sprint field from cloning, but this would be the best solution, since a sprint is planned within the sprint planning and is not used for defining by reporters.
The next problem is, that currently it is not possible to remove closed sprints by editing the issue. The only workaround provided by Atlassian is to use the bulk operation and remove the sprint id. But this is just a workaround and needs to be handled with care.
Thanks for any help or suggestions.