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To block an IP address or a domain name from accessing your website, put these lines in your .htaccess file.
You can specify a partial IP address (just the first three sets of numbers, for example) if you want to block a range of addresses.

You can also use a partial domain name.

Note that blocking by domain name depends on the browser sending the referrer code, which is blocked by some firewalls and browser configurations.

You can limit the blocking to certain files or directories by putting a <files> or <directory> tag around the code. When using the <files> tag you can specify a file name or a file type. See the example below:
You can expand this to block multiple IPs or domain names and create a special error page. Create and upload a special page, in this example banned.htm, then use this in .htaccess:
You can also use this technique to make a private website (or directory) by restricting access by IP address.
Using the .htaccess lines below, only requests from the specified IP address will be allowed:
More information can be found on these techniques in the Apache documentation here, at the bottom of that page.