How to prevent download page being accessible on a WordPress site?

In Wordpress I've created download posts for some of my products and created a category for downloads, which is specifically excluded in XML-sitemap-generator. I've also checked "no-index" for those posts.
My question is what things should I do actually to prevent the download posts from being found by search engine robots or not allow visitors access without login or paying first?
thanks in advance for any help with this.

You can do this quite a few different ways.

1.) Use a ticket system/ client management system.
You can find a tone of different ticket/client management systems out there on code canyon for very cheap prices or even maybe on the free wordpress plugin section.
https://codecanyon.net/search?utf8=✓&term=password+protect&as=1&type=p&referrer=search&view=list

Benefits:
Very manageable.
Allows for each user to have their own password.
Professional.
Allows banning, Restriction for countries and banning IP.
auto registration

Negatives
Tends to use a lot of resources.
Requires dealing with forgotten password issues.
May require you to keep up to date with security.

2.) Use .htacess
Just to have to set a few lines of code to restrict the page.
http://www.htaccesstools.com/articles/password-protection/
Benefits:
Fast and easy.
uses almost no extra resources.
cheap.
Allows banning, Restriction for countries and banning IP.

Negatives
No auto registration.
Not the most secure way.
Not easy to manage.
Not Professional.

To keep this short I leave you with these two ways, would love others to comment on more ways or recommend plugins/systems.

Interesting question if you didn't answer yet or for al others who want to know how to get download page on WP.

There are plugins called "membership"

Basic features are locking pages, tags, files, categories etc.
membership options
multi lvl membership

can't remember all features. There are free and paid.
I used one to make download page for some PDF's files, you can see it but you can't download it, unless you pay or you get membership status.
just search for membership plugins in wordpress.
If you need more details PM me

I recommend using .htaccess, there's plenty of tutorials to help you out with modifying the .htaccess, so you shouldn't have a problem.

Yes that can be done with .htacces and .htpasswd but then he must provide users with password, and who have password he can share on forums and have big traffic issue when all forum users start to download.

I'm still for membership plugin.