ah, ok, thank you for the details.
i was cursed once, never forget it, i was having a fight with someone who always threatened curses on people, one day i was having lunch and this terrible wave of negative energy hit me, i did a cleansing and protection, but since i knew who the person was i sent the energy back to him by charging it and visualising him being dragged down in tar. never has a problem with him since. you could try looking into a reversal/return to sender spell if you know the person in question. plus since you have a coven you can all cleanse your spaces and protect them and yourselves. [which is a bonus because you can conjure more energy in a group] daily shielding and wearing a protection charm can also keep the negative energies at bay. [you could also try a daily cleansing along with your shower, visualise negative energy being washed away as you scrub]
while i wouldn't recommend a group curse [because i don't curse is all] i would put it to a vote you either return the energy with a little extra push, or get together and curse him/her back. sometimes if they won't accept you asking to please stop, you have to use a little force. you could just protect and stay strong as a group [again, discuss your options together] but sometimes people will keep shoving you until you shove back, then they run away. best of luck to you guys.