By resigning with immediate effect, an employee is in breach of contract. The employer has a choice: Firstly, it may hold the employee to the contractual notice period. (In legalterms, this is known as ‘specific performance’.) Secondly, the employer may ‘accept’ the resignation, cancel the contract and claim damages.
An employer may proceed with disciplinary action and dismiss an employee before the contractual notice period expires. Employees therefore cannot escape the consequences of their actions by resigning with immediate effect. Hopefully this is the final word on the law in this regard.