The word "disservice" is formed by joining a prefix to another word in order to change its meaning. The prefix "dis-" means "not" or "the opposite of". It is changing the meaning of the noun "service". "Service" originally means doing something (good) for someone. A disservice is its opposite, meaning "doing a harmful action".
The correct spelling should keep both the -s belonging to "dis-" as well as the s- belonging to "service" - disservice.
