using System;
using Redmine.Net.Api;
using Redmine.Net.Api.Types;

namespace RedmineTest
{
    class Program
    {
        static void Main(string[] args)
        {
           string host = "<host>";
           string apiKey = "<api-key>";

           var manager = new RedmineManager(host, apiKey);

           int watcherIssueId = <watcher-issue-id>;
           int watcherUserId = <watcher-user-id>;

           manager.RemoveWatcherFromIssue(watcherIssueId, watcherUserId);

           Issue issue = manager.GetObject<Issue>(watcherIssueId.ToString(), 
                                new NameValueCollection { { RedmineKeys.INCLUDE, RedmineKeys.WATCHERS } });

           Console.WriteLine("Issue without the removed watcher {0}.", issue);
       }
    }
}

...
  await manager.RemoveWatcherAsync(watcherIssueId, watcherUserId);
...