Air purifiers actually pull the odor-causing bacteria and chemicals out of the air itself. They leave the air smelling fresh and clean. Air purifiers also get rid of other allergens in the air, making them suitable for those with allergies and asthma. These tend to trap odors. If the smell in your room is very strong, and keeps returning no matter how much you clean, there is a good chance that it might be trapped in the filters. Ozone generators produce O3, which oxidizes organic molecules (a common cause of smells). Many people find this an effective way of getting rid of smoke smells. Place the generator inside the room and set the timer. Make sure that all of windows in the room are closed. Vacate the room and close the door behind you. This will allow the ozone to fill the room. After the generator shuts off, wait at least one hour before going back into the room.  Ozone generators may irritate the throat. They are not recommended for those with asthma. They can actually make things worse.  Turn on the fan in the air conditioner in the room, if possible. This helps to circulate the air as well as clean out the air conditioner.  The stronger the smell, the longer you will have to set the timer. For example, if the smoker used the room for a few days, you'll need to run the generator for a few hours. If the smoker lived there for several years, you'll need to run the generator for a few days. Ozone generators can only remove the smoke smell to some extent. If the smell has sunk into the walls, floors, curtains, and furniture, you may not get the smell out all the way.

TL;DR in Vietnamese:
Mua máy lọc không khí. Thay tấm lọc trong máy sưởi và máy điều hòa. Sử dụng máy tạo khí ozone.