Hey ​@Martin_Aruachan1 ,
With the adminstrator, can you double check if you have accidentally added the clients to the project? If so, you’ll need to remove them in the Project Settings.

As a reminder, prototypes are created in files. Files can be grouped into projects. Projects are created within teams. On Organization and Enterprise plans, teams are grouped into organizations.

By default, each resource inherits access and permission levels from their parent. This means that a file will inherit the permissions set on the project level, and a project will inherit permissions set on the team level. More information in this article: Share files and prototypes.

Then, you should be able to limit viewers to the prototype only by going directly into the Prototype and sharing it from there (‘Share prototype’ button’):

 
As long as the Design file itself is set to "only those invited" users who are invited to prototype will not be able to view the file.
 
You can also set the prototype to "Anyone" can access then copy the link from the prototype to share it that way. Even with this setting they won't be able to access the main design file.
 
If you've already taken these steps and users are still able to access the main design file, feel free to reach out directly to the support team, so they can investigate it further here: https://help.figma.com/hc/en-us/requests/new?ticket_form_id=360005944714
Make sure to use your Figma email account, and please share a link to the design file and an example user (who shouldn't have access but can) so our support team can troubleshoot further. Thank you!
 

It looks like the recent update may have changed how prototype sharing works. You can try adjusting the link settings to "View-only" or "Prototype mode" to restrict access. If that option is missing, check the official documentation or support forums for any updates on sharing settings. If it's a bug or a new limitation, reaching out to support might be the best option.