Hi Marie Pinto,
There are two possiblities to remove columns.
1) Easier way: Either inherit view and just invisible those columns which you don't want to show to user.
2) little tricky: override def init(self, cr): method of this model and execute query to drop columns by altering table as suggest by vivek.
Risk factor in second step : If your are altering the table and removing any columns from table make sure those fields are not used in any views you have to remove it from there. if those fields are used in any base module view you have to replace it by inheriting views.
Hope this would clear more idea.
Regards,
Anil.