Vim: how do I clear all marks?

:delm[arks]!        Delete all marks for the current buffer, but not marks
            A-Z or 0-9.
            {not in Vi}

• 'A - 'Z uppercase marks, also called file marks, valid between files
• '0 - '9 numbered marks set from .viminfo file.

To delete them:

:delmarks A-Z0-9

Note: ~/.viminfo contains histories and marks, if you don’t want them any more, you can delete this file. Vim will recreate it next time.

So, you can do this to clear all marks:

:delm! | delm A-Z0-9