Yes just add a new property to your SitePageData model that's a boolean and then adjust the Search & Navigation code to only search for pages where it's set to true using the Match() method https://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-Find/11/DotNET-Client-API/Searching/Filtering/Boolean/ 

Thanks Scott for your suggestion, so I have gone ahead and added a boolean to our SitePageData.cs model:

        [Display(
            GroupName = SystemTabNames.Settings,
            Order = 351)]
        public virtual bool ExcludeFromSearch { get; set; }

Then in FindSearchPageController.cs I attempted to adjust the search by adding a filter as suggested in the example you posted:

        private ITypeSearch<ISearchableContent> BuildQuery(FindSearchContentModel model, string language)
        {
            var searchQuery = searchClient.Search<ISearchableContent>()
                .For(model.Query)
                .InField(x => x.Name)
                .InAllField()
                .UsingAutoBoost(TimeSpan.FromDays(30))
                .Filter(sp => sp.IsDeleted.Match(false))
                .Filter(x => x.ExcludeFromSearch.Match(false))
                .FilterForVisitor()
                .Filter(x => x.SearchLanguage.Match(language) | x.SearchLanguage.Match(string.Empty))
                .TermsFacetFor(x => x.SearchSection())
                .FilterFacet("AllSections", x => x.SearchSection().Exists())
                .TermsFacetFor(x => x.SearchTags)
                .FilterFacet("AllTags", x => x.SearchTags.Exists())
                .TermsFacetFor(x => x.SearchCategory)
                .FilterFacet("AllCategories", x => x.SearchCategory.Exists());

            if (!string.IsNullOrWhiteSpace(model.SectionFilter))
            {
                searchQuery = searchQuery.Filter(x => x.SearchSection().Match(model.SectionFilter));
            }

            if (!string.IsNullOrWhiteSpace(model.FilterByTags))
            {
                searchQuery = searchQuery.Filter(x => x.SearchTags.Match(model.FilterByTags));
            }

            if (!string.IsNullOrWhiteSpace(model.Category))
            {
                searchQuery = searchQuery.Filter(x => x.SearchCategory.Match(model.Category));
            }

            searchQuery = Sort(searchQuery, model.SortBy)
                .Skip((model.PagingPage - 1) * model.CurrentPage.PageSize)
                .Take(model.CurrentPage.PageSize)
                .ApplyBestBets();

            var doNotTrackHeader = System.Web.HttpContext.Current.Request.Headers.Get("DNT");
            // Should not track when value equals 1
            if (doNotTrackHeader == null || doNotTrackHeader.Equals("0"))
            {
                searchQuery = searchQuery.Track();
            }

            return searchQuery;
        }

The error I get however is: "ISearchableContent does not contain a definition for ExcludeFromSearch and no accessible extension method ExcludeFromSearch accepting a first argument of type ISearchableContent could be found"

If I look in ISearchableContent I see:

using EPiServer.Core;
using System;
using System.Collections.Generic;

namespace ProjectDigital.Site.Models.Search
{
    public interface ISearchableContent : IContent
    {
        DateTime SearchPublishDate { get; }

        IEnumerable<string> SearchTags { get; }

        IEnumerable<string> SearchCategory { get; }

        string TeaserText { get; }

        ContentReference PageImage { get; }

        string SearchLanguage { get; }

        IEnumerable<string> SearchBreadcrumbs { get; }
    }
}

Apologies I am new to this all. Do you have any suggestion for me perhaps? 

You need to add ExcludeFromSearch to the Interface.

using EPiServer.Core;
using System;
using System.Collections.Generic;

namespace ProjectDigital.Site.Models.Search
{
    public interface ISearchableContent : IContent
    {
        DateTime SearchPublishDate { get; }

        IEnumerable<string> SearchTags { get; }

        IEnumerable<string> SearchCategory { get; }

        string TeaserText { get; }

        ContentReference PageImage { get; }

        string SearchLanguage { get; }

        IEnumerable<string> SearchBreadcrumbs { get; }

       bool ExcludeFromSearch {get;}
    }
}

Thank you so much for your help Scott, I assume I will have to rebuild the find index for this to work correct?

Thanks so much Scott, working perfectly.