If someone tries to help, but their actions are not just useless, but actually damaging, is that "less than useless" or "more than useless"? Or would both be correct and understood in the same way?
The commonly used expression is "worse than useless."
"But arguments from the best of all worlds are always worse than useless."
"Drugs and pills are worse than useless, if anything they are harmful."
^ both quoted in Collins Dictionary
You could also use "less than useless":
They relieve the moviemaker of responsibility for his product and they encourage the viewer-juror to watch his own reactions to a film rather than the film itself, rendering his opinions less than useless.
His own father had been killed in an incident not more than a year after Yakov's birth--something less than a pogrom and less than useless....
^ Bernard Malamud, The Fixer
"More than useless" would be very atypical, unless it is part of a phrase such as "little more than useless," "no more than useless," or "nothing more than useless."