How to restrict column creation for existing tables? I have a table already created in a schema which has columns a, b, and c.  I want to restrict the user(s) from creating new columns.

How to do this?

I have gone through the manual pages about GRANT and REVOKE, but in the manual it is explained only for INSERT, UPDATE, SELECT, etc.

I want to de something like

REVOKE ALTER on Table schemaname.tablename  FROM user;
The way to prevent table modification is to change the ownership to a different role.  Tables can be altered only by the owner and superusers - so change the ownership to a non-superuser.  I find it practical to keep a database ownership role for solely this reason (which might make life easier elsewhere, too, like for example when setting default privileges).