Things can happen if you upset a spirit or didn't say goodbye it could stay with you, either for companionship or to bother you.
Easiest way to end a session is say goodbye and wish it a safe journey home.
The spirit usually leaves after a bit depending on how it feels, but you would sense it in the room after you end the session. Could be an energy, or the feeling of being watched. You might also see it as a shadow or orb in the room. It could be your imagination if you want it to still be in your house, but when I use the board and speak with a wandering spirit, and they stay, I say they can leave in the morning, and they do. If the spirit overstays its welcome, a cleansing and protection will 'kick' the spirit out of your house like any unwanted house guest.
Like with a spirit overstaying its welcome, you can protect yourself after the session by cleansing the house [sage smudge or salt water work] then protecting it. Be sure to always cast a circle, this will keep negative spirits away. However, the board is like a spirit magnet, once you begin a session, any spirit in the area is pulled towards it. Once you open the circle, a negative spirit could be waiting, so cleanse the room, and yourself before and after using the board. It's also a good idea to cleanse the board every so often depending on how often you use it.