If "belong" means to be a part of something:
* Not belong: This is the most direct opposite.
* Be excluded: This emphasizes being kept out.
* Be an outsider: This suggests a feeling of not fitting in.
* Be alien: This implies being completely foreign or different.
* Be estranged: This suggests a separation from a place or group.
If "belong" means to be in the right place:
* Be misplaced: This suggests being in the wrong place.
* Be out of place: This emphasizes feeling uncomfortable or unsuitable.
* Be lost: This implies being without a sense of direction or purpose.
If "belong" means to be owned by someone:
* Be unclaimed: This suggests not being owned or possessed.
* Be abandoned: This implies being left behind or forgotten.
* Be free: This suggests being independent and not under anyone's control.
So, the best opposite of "belong" will depend on the specific situation and the meaning you want to convey.