Delete object from array by given _id in in Mongoose
Quote from SERVER-2016:
The argument to $pull is already applied to each element of the target array, so using $elemMatch is redundant in the update context.
So just execute your query without $elemMatch:
User .findByIdAndUpdate(req.user._id, { $pull: {events: { _id: _eventId //_eventId is string representation of event ID }} }, function(err) { // ... });