В каком месте должен быть код?

Запрет доступа по HTTP заголовку referer для блокировки нежелательного входящего трафика:

RewriteEngine On

Options +FollowSymlinks

RewriteCond %{HTTP_REFERER} ^https?://([^.]+\.)*semalt\.com\ [NC,OR]

RewriteRule .* – [F]

конкретно в каком месте файла .htaccess должен стоять?

RewriteEngine On

########## Begin - Rewrite rules to block out some common exploits

## If you experience problems on your site block out the operations listed below

## This attempts to block the most common type of exploit `attempts` to Joomla!

#

## Deny access to extension xml files (uncomment out to activate)

#<Files ~ "\.xml$">

#Order allow,deny

#Deny from all

#Satisfy all

#</Files>

## End of deny access to extension xml files

# Block out any script trying to set a mosConfig value through the URL

RewriteCond %{QUERY_STRING} mosConfig_[a-zA-Z_]{1,21}(=|\%3D) [OR]

# Block out any script trying to base64_encode data within the URL

RewriteCond %{QUERY_STRING} base64_encode[^(]*\([^)]*\) [OR]

# Block out any script that includes a <script> tag in URL

RewriteCond %{QUERY_STRING} (<|%3C)([^s]*s)+cript.*(>|%3E) [NC,OR]

# Block out any script trying to set a PHP GLOBALS variable via URL

RewriteCond %{QUERY_STRING} GLOBALS(=|\[|\%[0-9A-Z]{0,2}) [OR]

# Block out any script trying to modify a _REQUEST variable via URL

RewriteCond %{QUERY_STRING} _REQUEST(=|\[|\%[0-9A-Z]{0,2})

# Return 403 Forbidden header and show the content of the root homepage

RewriteRule .* index.php [F]

#

########## End - Rewrite rules to block out some common exploits

########## Begin - Custom redirects

#

# If you need to redirect some pages, or set a canonical non-www to

# www redirect (or vice versa), place that code here. Ensure those

# redirects use the correct RewriteRule syntax and the [R=301,L] flags.

#

########## End - Custom redirects

# Uncomment following line if your webserver's URL

# is not directly related to physical file paths.

# Update Your Joomla! Directory (just / for root)

# RewriteBase /

########## Begin - Joomla! core SEF Section

#

RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]

#

# If the requested path and file is not /index.php and the request

# has not already been internally rewritten to the index.php script

RewriteCond %{REQUEST_URI} !^/index\.php

# and the request is for root, or for an extensionless URL, or the

# requested URL ends with one of the listed extensions

RewriteCond %{REQUEST_URI} (/[^.]*|\.(php|html?|feed|pdf|raw))$ [NC]

# and the requested path and file doesn't directly match a physical file

RewriteCond %{REQUEST_FILENAME} !-f

# and the requested path and file doesn't directly match a physical folder

RewriteCond %{REQUEST_FILENAME} !-d

# internally rewrite the request to the index.php script

RewriteRule .* index.php [L]

#

########## End - Joomla! core SEF Section