I think the interval values need to be quoted. In any event I always use the P or T syntax. 'P1D' means add one day,
unfortunately, I can not call INTERVAL 'in_until', that wouldn't work.
Also 'P1D' vs. '1 day' seems to be just a matter of taste.
Finally, to provide more context to my question -
I would prefer to call my custom function as
select words_ban_user(1, '1 day', 'attacking other users');
and not as
select words_ban_user(1, CURRENT_TIMESTAMP + '1 day', 'attacking other users');
because additionally to the temporary ban I would like to prolong VIP-periods for paying users (to avoid discussions) - and that would be more difficult in the latter case (would require more date acrobatics)...