You can prevent un authorised access by giving a password to open the
workbook itself, so that no one else can open it (Go to Tools, Options and
then Security Tab). You can do it there if you have already saved the file,
or you can go to "save as" then "Tools" and "General Options" where you can
give password.
On the other hand you can protect work sheets and ranges for different
people by using "Tools" and then "Protection" Tab