Hi @Christian Ebert ,
Please follow steps:
1.Sign in List as an admin, click on Library settings under the Setting menu

2.Go to Library Settings page, click on Advanced settings, enable "Allow management of content type" option.

3.Go back Library Settings page, scroll to Content Types section, and then click on "Change new button order and default content type".
4.Go to Change New Button Order, just keep the Content Type you wanted and cancel other Content Types. For example, I only keep the Document Content Type and cancel other Content Types.

5.Go back List and go to "Edit Properties" panel, the Content Type drop-down box only displays you just selected and reserved the Content Type.

Thanks,
Echo Du
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