Good day 

If you signed an employment contract which states that you must work the notice period you are bound by it or if there is a collective agreement in effect. The Basic Employment Conditions of Employment Act sets out the prescribed notice period. if you do not work this notice period an employer may sue you for the month in which you did not work. If you are intending to leave a few hours earlier on the last day of your notice period and the employer threatens to sue you this would give you rights to approach the department of Labour. This is due to the fact you already worked the stipulated 30 days notice period as per the BECA. 

Regards 

LP