Can spirits possess people through ouija boards?
while i've never encounter someone being possessed, it can happen. a Ouija Board is a portal, or a spirit magnet, it draws any being towards it. the person [persons] are then used by the spirit to move the planchette.
Can spirits take things from people in exchange for certain services?
not 100% what you mean. can a spirit take, say, half your life, no. sometimes they'll do it in exchange for an item, or something from the human, like energy, sometimes they ask for a binding so the living person is bound to the spirit, the living gets more power and the spirit gets more energy to survive in this realm.
can spirits do things for people in exchange for certain services?
kind of the same as the above question. but, yes, sometimes a spirit can exchange items for a task. example, a ring in exchange they will protect you from another person. don't know how often this is done though.
During meditation or rituals, can spirits influence anything if uninvited?
if uninvited? no, but people do subconsciously influence spirits. unless you say 'no spirit can disturb me' then while a spirit is wandering around your house, they can mess with you. granted, everyone has a shield up, so not many spirits can break through it and, say, enter into your mind while you meditate, but unless you make it clear no spirit is aloud anywhere near you, spirits do enter your home. entering your mind is another issue. rituals, only if you haven't cast a circle. i've dealt with many spirits, and i've noticed, good or bad, spirits get stuck outside the circle unless you wait for them to enter before casting. there's been many times i've cast a circle, then tried to talk to a spirit friend of mine, only to realize they didn't know we were casting the circle and got stuck just outside, so we'd open the circle, wait a minute, then close it again. unless you summon them, or call upon them to join you in your circle, they can't effect your ritual, or meditation, or whatever else.
I heard of some people saying they had made certain deals with certain spirits to help them with their spell.
as i've said before, yes, it can be done. normally, though it's like calling upon the Lord and Lady, you ask them to add their energy to a spell, then you leave an offering of some kind. same goes for spirits. in exchange for a spirits help, you can burn some incense, or leave something for them to have. with faeries people will leave out sweets, honey, milk or cakes. with dragons, depending on the dragon, coins are left, or meat. it's a thanks for helping.