Hi @Bart De Keyser ,
In SharePoint online you could set unique permission for folder or files by "Manage access" option.
1.Select your folder > click the ellipsis button > Manage access
2.Click Advanced
3.Click Stop Inheriting Permissions then you could set folder unique permission grant or remove user permission.
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.