I've had a few of those pissy little babies downvote me in the past. I've found they generally get tired and go away after a while on their own, or after they are themselves downvoted out of significance. They don't seem to realize that it may give only give them some minute amount of short-term revenge pleasure. Like most things that enjoyment quickly fades out of their lives, becomes more work on their end, and it will eventually get pulled into their own little black hole of misery.
Watch and wait, go about your business posting and let them fade away, or move to other platforms where they can get the attention they so desperately seek. I find them to be like little 3 year old children. If they don't get enough positive attention, they'll try for negative attention out of desperation, rather than just leave the group and go elsewhere to find enjoyment like an adult.