Yes! :-)

Have a look at the documentation for custom for actors, and notice that you are able to cancel a form submission and display an error message. 

https://world.episerver.com/documentation/developer-guides/forms/implementing-a-customized-actor/

I did something simular by creating a custom actor to detect URLs in our form which you could modify to check for emails

  public class FilterURLActor : PostSubmissionActorBase, ISyncOrderedSubmissionActor
    {
        public int Order => 1;

        public override object Run(object input)
        {
            var result = new SubmissionActorResult { CancelSubmit = false, ErrorMessage = string.Empty };

            foreach (var i in this.SubmissionData.Data)
            {
                if (FormsHelper.HasURL(i.Value.ToString())  && i.Key.ToString().Contains("field")  && !i.Value.ToString().Contains("@"))
                {
                    result.ErrorMessage = "Error: URL in form.";
                    result.CancelSubmit = true;     
                }
            }
            return result;
        }

    }

I'm using a helper class called FormsHelper.HasURL(i.Value.ToString())  which I use to detect a URL.   I aslo have one for emails - this is the code I'm using. I know the regex for emails are... fun

        public static bool isProperEmail (string s)
        {
         
            s = Regex.Replace(s, @"\A(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?)\Z", "[[Email]]");
            if (s.Contains("[[Email]]"))
                return true;
            else
                return false;

        }

Anyway, this probably isn't the most elegant solution but it works.