CancelBuild.cs

using System;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.TeamFoundation.Build.WebApi;
using Microsoft.VisualStudio.Services.Common;

class Program
{
    static void Main(string[] args)
    {
        CancelBuild("https://<yourdomain>.VisualStudio.com/DefaultCollection/", "<username>", "<password>", "<project>", "<BuildDefinitionname>").GetAwaiter().GetResult();
        Console.ReadLine();
    }

    static async Task CancelBuild(string url, string userName, string password, string project, string buildDefinitionName)
    {
        var build = new BuildHttpClient(new Uri(url), new VssBasicCredential(userName, password));

        // Get the specified name build definition to get the id of build definition
        var definitions = await build.GetDefinitionsAsync(project: project);
        var target = definitions.FirstOrDefault(d => d.Name.StartsWith(buildDefinitionName, StringComparison.InvariantCultureIgnoreCase));
        if (target == null)
        {
            throw new ArgumentException($"No build definition with name: {buildDefinitionName}", nameof(buildDefinitionName));
        }
        //Get the builds of the specified name build definiion
        var builds = await build.GetBuildsAsync(project, new[] { target.Id });
        //Update the builds with the status other than "Completed" and "Cancelling" to cancel status
        foreach (var b in builds.Where(b => !(b.Status == BuildStatus.Completed || b.Status == BuildStatus.Cancelling)))
        {
            b.Status = BuildStatus.Cancelling;
            var res = await build.UpdateBuildAsync(b, b.Id);
            Console.WriteLine($"Cancelled build id {res.Id}");
        }
    }
}