echo 0.0.0.0 websitename.com >> %WINDIR%\System32\Drivers\Etc\Hosts
the >> appends the output of echo to the file.
Note that there are two reasons this might not work like you want it to. You may be aware of these, but I mention them just in case.
First, it won't affect a web browser, for example, that already has the current, "real" IP address resolved. So, it won't always take effect right away.
Second, it requires you to add an entry for every host name on a domain; just adding websitename.com will not block www.websitename.com, for example.
Use Hosts Commander. It's simple and powerful. Translated description (from russian) here.
hosts add another.dev 192.168.1.1 # Remote host
hosts add test.local # 127.0.0.1 used by default
hosts set myhost.dev # new comment
hosts rem *.local
hosts enable local*
hosts disable localhost
...and many others...
Usage:
hosts - run hosts command interpreter
hosts <command> <params> - execute hosts command
Commands:
add <host> <aliases> <addr> # <comment> - add new host
set <host|mask> <addr> # <comment> - set ip and comment for host
rem <host|mask> - remove host
on <host|mask> - enable host
off <host|mask> - disable host
view [all] <mask> - display enabled and visible, or all hosts
hide <host|mask> - hide host from 'hosts view'
show <host|mask> - show host in 'hosts view'
print - display raw hosts file
format - format host rows
clean - format and remove all comments
rollback - rollback last operation
backup - backup hosts file
restore - restore hosts file from backup
recreate - empty hosts file
open - open hosts file in notepad
Source: Stackoverflow.com