To disable/enable cell protection settings for a given worksheet, use the Tools > Cell Protection command. Your password must consist of at least 6 characters.

Once the cell protection is active, you can use the Format > Protected command to turn on/off the individual cell protection. Protected cells can not be deleted.
Each time you (un-)protect some cell(s), you'll be asked to supply a password defined as explained above.

Note: Unless your workbook is encrypted at the same time, a person having full access to your file can use another software capable of reading/writing *.ods files to change this settings.