preventDefault() on the contextmenu event can be used to prevent right click in Salesforce Lightning Web Component.

Sample Lightning Web Component:

HTML:

<template>
    <lightning-card>
        <div class="slds-m-around_medium">
            Testing
            <lightning-input
                label="Text"
                type="text">
            </lightning-input>
        </div>
    </lightning-card>
</template>

JavaScript:

import { LightningElement } from 'lwc';

export default class SampleLightningWebComponent extends LightningElement {

    constructor() {

        super();
        this.template.addEventListener(
        "contextmenu", 
        event => event.preventDefault()
        );

    }

}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>60.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__Tab</target>
    </targets>
</LightningComponentBundle>