@Steve After encountering the same issue and after some research I found there is no longer any independent way of factory resetting a Phillip Hue lamp, other than with access to a Phillips accessory (a bridge or a remote). The least expensive method involves purchasing a Hue remote and following the guideline given in this video: https://www.youtube.com/watch?v=qvlEAELiJKs
@emiespo I’m also controlling my Phillips Hue lamps primarily through an Echo Plus. An interesting phenomena you describe with the Echo capturing a neighbours bulbs. I’d surmise that a scenario that may go to explaining this would be that if the neighbour may have had some factory reset or virgin bulbs sitting active at the time your Echo commenced its pairing cycle. This may not be as odd as it would at first appear, as with close inspection these bulbs don’t look a whole lot different than a ‘normal’ LED bulb. The neighbour may simply have been using the lamps as static on/off devices.
As to recovering control of your own original bulbs, cheapest & least time consuming path I suggest is as above – purchasing a Hue remote.