well I would recommend doing it physically, but in some cases bonds are unbreakable completely physically, as they may stop seeing each other or live apart, but karma runs deep, if this is the case, you can raise a shield around the one who is being negatively influenced by the other which cuts and repells the cord by which the people exchange energy, which is an unconscious thing we do when karma runs deep. The shield dhould cut off exchange of energy, but only between the two.
I believe that communication is essential when dealing with bad influences. I suggest that you address your concerns to both parties. Express to the party being negatively influenced that you are concerned about them and you care; letting them know that you are not okay with their behaviors and actions and how it affects the people around them. In addition talk to the person or people that are being a bad influence and let them know what they are doing and how it is affecting the other person/s and how it affects you and that it is not okay.
In addition there are other methods such as doing some simple workings such as bindings, sheilding, perhaps a tranqulity or peace spell or ritual as well as a banashing of negativity. Smudging to purify and clear out negativity is great. Also there are some stones with magical properties that can be helpful such as blue chalcedony to help communicate and clear negaitvity, also black tourmaline is a great banashing and binding stone for repelling and getting rid of anything unwanted including negativity. Ukanite Jasper helps to rid people of bad habits, behaviors, and influnces and also helps to promote healthy relationships. And their are many other stones with the same and similar properties to help like rose quartz for healing. Using herbal magic there are many powders and brews that can also be used to rid negativity and bad influences. Good luck.