It is not permissible

To say that it is not permissible to do something does not necessary mean that it is Haraam. If something is Makruh-e-Tahreemi, then too it is not permissible to do it. Similarly, we can say that it is not permissible to do something that goes against the Mubaarak Tareeqah and Sunnah of our Nabi (Sallallaahu Alaihi Wasallam). Above all, if a principle or a Sunnah of Rasulullah (Sallallaahu Alaihi Wasallam) is neglected regularly, this is even more grave. This is because to make it a habit to neglect a Sunnah indicates a type of affront. That is why this is also not allowed just as it is not permissible to commit Haraam. It is true that committing Haraam is more grave than neglecting a Sunnah. Nonetheless, just as the one is not permissible, the other one also is not permissible. 

[English version of "Pas gagne droit faire ene kitchose ki contre le sounnah de nou Nabi ‎(Sallallaahou A'laihi Wasallam)" – Al Jamiat – Vol.2 No.3]