What do you mean by another type of advertising?
Like making promotional videos and blogs which some of the bounties have and doesn't have it. Actually there are many ways of advertising online as long as you use the legal way.
How legal will it be to stop the bounty program? This is likely to be a serious damage to the reputation of the project.
It's a project's discretion and IIRC there's no legality if you want to stop. It's like that you just want to stop your advertisement so there's nothing wrong if you find its not helpful anymore and its not effective to the promotion as long as you'll pay the participants that joined your program.