Protect a plugin from being copied and used in another site

Create a blank text file and upload it to your wp-content folder (Unless you have one there already). Rename it .htaccess and add the following code:

# Deny access to .php files
<Files *.php>
deny from all
</Files>

You could also add the following code to your main .htaccess file in your root (Back up the original first!!! Then place the code above the existing # Begin WordPress rules).

# PREVENT FOLDER BROWSING
Options All -Indexes
Options +FollowSymLinks

Edited

Thanks Barnez but I’m completely familiar with htaccess.
I mean Is there any way to protect the source code from the site owner or other people that have access to the Host?

I see.
This thread about requiring an API key could offer a possible solution:

https://wordpress.stackexchange.com/questions/37011/how-do-i-protect-my-premium-wordpress-app-theme-from-copying