my experience is you can terminate the contract whenever you like... just pay 1 month salary + other compensation to her and let her go... NO need warning letter (coz I haven't issued any warning letter to her!)
Unless you found she had committed crime... or something very obvious ... e.g. hurt your bbs (with wound which can be seen.) or add something into the food which make u all lulu (witht he evidence)... or steal your things (with evidence)... you can fire her immediately without compensation.