How to disagree when someone says 没关系 or 没有意思?

Hi everybody,

I have found a few situations where I am not sure how to reply to 没关系 and 没意思.

For example:

没关系的 (It's no problem)

I can't simply say 有关系的, right?  What is the simplest, most natural and direct way to say "It is a problem"?

做这样没意思 (Doing it this way is pointless)

Can I simply say 有意思 if I disagree?  I feel that it isn't appropriate here, but am not sure.  What's the natural way to say it?

Thanks!