I found the way to remove the footer link,

By simply Hiding with a frame,

in this case a backlink to yaf is active,

but not shown on my forum

Is that ethical or shall i revoke that.

I wanted this for commercial purpose but was unable to pay for that

It is definitely unethical in my opinion. With everything the software has to offer, as well as the time involved for the developers to maintain such a project, it's a matter of courtesy to leave those links in, unless you are able to cover the fee to purchase a 'copyright/link removal key'. In my opinion, this is especially unethical if the software is being used in a commercial environment. I am not speaking for the YAF Team or Developers though -- just as a member of the community who uses the software as well and does what I can to help support it.

Jaben may wish to reply to this, but according to the YAF Documentation  you are permitted to remove the logo.

-Dave

Update: Squirrel was posting a reply while I was, so let me update my post to say that I agree with his opinion 100% on the ethics of removing YAF information.