Guenter,
Depending on the number of users and whether you want to lock them out once or regularly
...or really just lock ftp for them you might want to look at "The lazy admin tools".
Lazy Admin Tools HowTo
http://mirror.contribs.org/smeserver/contribs/mblotwijk/HowToGuides/lazy-admin-tools.htmUsing LAT a small script:
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#!/bin/sh
lat-users -a -c harry | Harry | Potter | Quidditch''
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Creates user 'harry' with password 'Quidditch' from the command line.
or:
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#!/bin/sh
lat-users -d -f -c harry''
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Deletes user 'harry' from the command line.
If you place such a make-harry or kill-harry into the daily or weekly crontab you could open & close access for harry.
HTH
Reinhold