Do Sikhs practice shunning or excommunication?

Answer:
Yes and No.

No: Sikhs do not practice shunning anyone based on religious creed, social ranking of caste, or race including skin colour. Sikhs do not engage in shunning by refusing to speak to an offender.

Yes: The Sikh Code of Conduct, Sikh Rehit Maryada (SRM), specifies certain conditions which bar association with perpetrators resulting in boycott, and if not rectified, could ultimately result in excommunication:

Barring and Boycott

Sikhs are to boycott any person, physician, parent, or relative, involved in gender selective abortion of an unborn female foetus, killing of new born infant girl, as well as anyone who has participated in any kind of honour killing of a daughter for marrying without parental permission, having premarital or extra marital relations, or any one having forced a widowed daughter to Sati meaning to immolate herself on her deceased husbands funeral pyre, all practices common in the past, or present, to India, and other countries in the Middle East and Asia.
Sikhs are advised not to befriend thieves, thugs, gamblers, or any who make a living by dishonest means, including those who deal in illegal contraband, engage in exploitation, prostitution and slavery, or other questionable actives.
As well as the above prohibition, baptized Amritdhari Sikhs, who have been initiated into the Khalsa order, may be subject to boycott for breach of conduct including:•Dishonoring hair by coloring, curling, cutting or other means of removal.
Smoking or use of other intoxicants including hemp products, opium or other narcotics, stimulants such as cocaine, and any type of liquor.
• Breaking dietary law including sharing of food and drink, or eating leftovers, from the plate of an offender.
• Marital arrangements made for profit, of a son, or daughter, including forced marriage.
• Premarital or extra marital affairs. Any sort of relationship with Muslim women for whom breaking Islamic Sharia Law including infidelity, rape and even marriage to a non-Muslim man, could result in severe penalties such having her nose cut off, or be punishable by whipping, hanging, stoning, beheading, or execution by a firing squad.
• Involvement in ceremonies and ritualistic practices which are not in accordance with, or which do not conform with, Gurmat, Sikh ideals and way of life.
• Involvement with persons, parties, or politics in opposition to Sikhism, or adherents of Sikhism, including religious affiliation with devotees of Dhir Mal or Ram Rai who reject, or seek to usurp, Guru Granth Sahib as the successor of Tenth Guru Gobind Singh and Eternal Guru of the Sikhs.
• Keeping company with unrepentant fallen Sikhs guilty of ongoing willful and deliberate misconduct including all of the above.
The bar for breach of conduct is in effect until such time as the fallen Sikh, known as a Patit, has requested and carried out penance, or Tankhah, and Amritdhari status is reinstated.

Every charitable effort is lovingly carried out to encourage ultimate repentance and reinstatement of one who has been subject to boycott because of misdeeds. However hanging out socially is discouraged where there is possibility of adverse influence, or of any occurrence which could compromise code, or conduct, of another resulting in the consequence of boycott.

Restraint of association in no way applies a ban from worship services, or involvement in seva, with the exception of food preparation, most especially cooking food meant for sangat, and preaching against Sikh tenets.

Excommunication
Excommunication is rare in Sikhism but does occasionally occur in extreme cases. This usually involves a perpetrator preaching or public speaking against Sikh ideals within a gurdwara or worship service. If a boycott which has been placed barring the speaker is not respected, and all other efforts at rectification fail, the offending party may be called to appear at the Akal Takhat in Amritsar, India, before Panj Pyare, a court of five Amritdaris, for penance. If the perpetrator fails to appear, they may be excommunicated. Reinstatement is always an option.

An offender is always to be greeted respectfully, never belittled or avoided by turning away or subjected to hurtful actions, but treated humanely with utmost decency and kindness. This does not mean any misconduct, or harmful behavior, is to be condoned, accepted, or tolerated, nor ought any disparagement dictate a like response, or spiteful retaliation even should opposing parties stage a protest, or meet on the battlefield. Rather love, compassion and humility are to be ever present in the heart and mind, and on the tongue of a true Sikh, when dealing in both communication and excommunication.