Sikhs should not back-bite:
[INDENT]
Great is the Greatness of the Lord; He does not hear the words of the back-biters.(Guru Granth Sahib, Ang 84)
By slandering, the slanderer washes the filth off others; he is an eater of filth, and a worshipper of Maya (the materialistic world).(Guru Granth Sahib, Ang 506)[/INDENT]
Sikhs should not hurt other people's feelings:
[INDENT]
Do not utter even a single harsh word; your True Lord and Master abides in all.
Do not break anyone's heart; these are all priceless jewels. ||129||.(Guru Granth Sahib, Ang 1384) [/INDENT]
Sikhs should not slander other religions:
[INDENT]
Do not say that the Vedas, the Bible and the Koran are false. Those who do not contemplate them are false.(Guru Granth Sahib, Ang 1350)[/INDENT]
Sikhs should not use bad language:
[INDENT]
O Nanak, speaking insipid words, the body and mind become insipid.(Guru Granth Sahib, Ang 473)
That soul-bride, who talks sweetly and speaks the Truth, O Siblings of Destiny, becomes pleasing to her Husband Lord.(Guru Granth Sahib, Ang 637) [/INDENT]
Sikhs should remain true to their word, and not be "two faced":
[INDENT]
He gives his word, but then backs out. All that he says is false. The false person is hollow within; he is totally engrossed in falsehood.(Guru Granth Sahib, Ang 1099) [/INDENT]
Sikhs should not spread rumours about others:
[INDENT]
Watching, watching the transitory dramas of the world, you are embroiled and enmeshed in them, and you laugh with delight. The string of life is wearing thin, day and night, and you have done nothing for your soul. ||2||.(Guru Granth Sahib, Ang 206) [/INDENT]
There are many more quotes on this sort of behaviour, but I think you get the idea