Hey, najeem.
The END button is a known issue of nimbuzz. but sorry to tell you: not a resolution for this bug yet.
The Nimbuzz team are aware of this issue and they expect to have it solved in upcoming release.
Someone suggests to use the physical end call button to terminate the call.
But Nokia N8 has no physical End button in the phone. and there is no end button or mute button available in Symbian belle.
So, now the only way is to let the other people handed up the phone and the call is disconnected.
Questions at the same topic:
http://ask.brothersoft.com/not-available-end-buttan-pls-help-pls-pls-pls-249043.html
http://ask.brothersoft.com/pls-help-me-my-mobile-is-nokia-n8-symbian-belle-nimbuzz-functionality-good-experiance-but-now-not-available-end-buttan-248981.html
http://ask.brothersoft.com/pls-help-me-my-mobile-is-nokia-n8-symbian-belle-nimbuzz-functionality-good-experiance-but-now-not-available-end-buttan-248941.html