Delete all fork repositories from your Github account in one go, using this Golang script :)

client.go

/*
 * CAUTION!!! READ THIS VERY CAREFULLY!!!
 * This will delete ALL your Github fork repositories.
 * DO NOT use it if you don't know what it does!
 *
 * Usage:
 * $ go get # get dependencies
 * $ go run client.go # initiate chaos
 */
package main

import (
	"fmt"
	"log"

	"github.com/google/go-github/github"
	"golang.org/x/oauth2"
)

var allRepos []*github.Repository

const (
	user        = `` // change that to your Github username
	accessToken = `` // enter your Github access token here (need delete permissions)
)

func main() {
	ts := oauth2.StaticTokenSource(
		&oauth2.Token{AccessToken: accessToken},
	)
	tc := oauth2.NewClient(oauth2.NoContext, ts)

	client := github.NewClient(tc)
	opt := &github.RepositoryListOptions{
		ListOptions: github.ListOptions{PerPage: 50},
	}

	// get all pages of results
	for {
		// list all repositories for the authenticated user
		repos, resp, err := client.Repositories.List("", opt)
		if err != nil {
			log.Fatalln(err)
		}

		allRepos = append(allRepos, repos...)

		// reached last page of results, exit the loop
		if resp.NextPage == 0 {
			break
		}
		opt.ListOptions.Page = resp.NextPage
	}

	for _, repo := range allRepos {
		if *repo.Fork {
			_, err := client.Repositories.Delete(user, *repo.Name)
			fmt.Println(*repo.FullName, "is now deleted!")
			if err != nil {
				// We're not using log.Fatalln() here because if you re-run
				// this program, Github will return cached response with deleted
				// repositories, and repository deletion will fail.
				fmt.Println(err)
			}
		}
	}
}