Hi Pablo,
1. By default, entries can only be edited by their author, or by someone who has a higher level of permissions (e.g. a teacher in the course). You can prevent students from creating new entries, by forcing 'approval' for new entries, and never approving them. There are other ways, such as changing permissions for this activity, but it's a little bit more complicated (it is, however, a better solution). You can find permissions in the 'Administration' section of the database activity you have created (it's usually in a block when you are editing the activity).
2. If you use a 'textarea' field type, you should be able to input a table. Having said that, I have just noticed that the 'table' icon on the Atto editor doesn't work. I am not sure whether that is a bug or a feature, but you *can* create tables by copying and pasting HTML directly into the editor (using the <> icon).
Cheers,
Fred