I have read and understand 6 months probationary period rule, that employee exceeds 6 months of probationary period shall be regularized otherwise informed by the employer to not continue. Now the question is employee is informed ahead that he/she will be regularized, however wished to decline because he/she doesn't agree with the new contract/rules. Now, does the employee has the right to do so?
Yes, the employee can choose not to accept the contract offered at the end of the probationary period and thereby allow the probationary period to end without accepting a long-term position.
This is usually called "quitting".