We would like to exclude a specific issue type from sprint creation.
We are planning to use a issue type "Idea" to keep track of possible improvement. This type is not design to be put in a sprint but to act as a reminder. Is that possible to forbid this ?
Your board is based on a filter, if you exclude your issue from the filter ( issuetype != Idea ) it will not show up.
You can change the filter based on this documetation https://confluence.atlassian.com/jirasoftwarecloud/configuring-filters-764477988.html
I was using this to hide subtasks in a sprint since we only use them as a trackable checklist for a ticket. Messing with the board's filter prevented those subtasks from showing on the board, so that made me happy!
As for a complete avoidance, I'm not sure how you would prevent it from coming into the sprint other than setting up a permissions/security level for the issue type. If only admins or managers can edit the issue type, then that's less people that accidentally put it into a sprint. You can learn more about security levels here: https://confluence.atlassian.com/adminjiraserver/configuring-issue-level-security-938847117.html
The only other option I can think of is making a separate board for these Idea issue types. You could make it a Kanban board and then always have those ideas on display for people to look at, that way it never goes into a sprint. Then you can remove the issue type from any other project so no one can accidentally make it either.