"You've been very helpful", by itself, would be a positive comment that you appreciated the help. It would not necessarily imply that you didn't need or want the help. "You've been very helpful, [but] I can do this" means "thanks for the help, but I don't really need it." It would be taken as a negative comment. If you didn't want the negative tone, you would say: "Oh, that's all right. I've got it/ I can get this. Thanks, anyway". If a person had tried to help, but actually made things worse, then replying in a sarcastic tone "Gosh - thanks for the help." would be completely negative. Finally, "You're so helpful!" would be very positive - you needed the help a lot and the person made an extra effort in helping you.

The difference between your first two sentences is simply one of tense, not meaning. Your further example, starting with "no thanks'' indicates that the help given is in the past, so the speaker uses that tense. The English language pays strict attention to tense and number, to a much greater extent than does the Chinese language.