You can change the username portion of your default address.

Just click on the "default address" link in your control panel and type in the new address. 

Type the entire address: yourname@yourdomain.com.

Then click "change".

Make sure you are using an account that actually exists. If the account doesn't exist, set it up using the new email accounts feature.

You never have to create email aliases. 

Your default email account is setup as a "catch all" account, meaning that any email coming to anything@yourdomain.com is delivered to your default account.

This tool is also useful if you wish to have all incoming email (or all undeliverable mail) sent to another email address such as a Yahoo or Hotmail address. 

The Webmail Alternative: 

You can also check your default email account, or another other mail account by logging into it through the "WebMail" interface.

 Simply select the "WebMail" icon at the bottom of C-panel, and log in to it using your "Main Account" Username and Password.  

This will allow to to check your default email box, as well as other mailboxes without having to configure them in your mail reader.  

In fact, using any pop accounts "Username and Password" will log you into that particular account through the "WebMail" interface.

Disabling your "Catch All Feature"

Instead of entering a syntax legal name, use illegal syntax, which will effectively disable your email "catch all." 

For example, using characters, which are known as 'illegal' to the email system such as (>>>????) will work just fine.  These are characters, which cannot be used in an email address, which in effect, will render the "Catch All" feature useless.  Go to your "change default email address" and add something like the above as default name.

What happens now?

When Spammy or Jimmy Junk Mailer attempts to use a random email address to Spam you, it will be bounced back to them. That is, unless they happen to get a hold of one of your "legitimate pop email account names", in which case, you'd have a different problem on your hands. Yes, you could either deal with it, or change the address.
 

Here is what now happens to a sender using anything@yourdomain.com :

This is what the sender would receive. Please note that a classic, but annoying junk mail example is being used here:

This message was created automatically by mail delivery software (Exim).

A message that you sent has not yet been delivered to one or more of its
recipients after more than 24 hours on the queue on yourdomain.com.

The message identifier is: 14m7gv-0007gl-00
The date of the message is: Mon, 04 June 2001 01:23:02 -0400
The subject of the message is: MAKE MILLIONS FAST!

The address to which the message has not yet been delivered is:

anything@yourdomain.com

Delay reason: error in alias file

/etc/valiases/anything@yourdomain.com:

missing or malformed local part (expected word or "<") in "******>>>" (Bad email syntax)

No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you.

So what actually happened here?

When the "Catch All" email address

(******>>>@yourdomain.com), 

attempted to process an incoming message from anything@yourdomain.com, and then forward the (junk message in this case) to the "catch all/Default" email address, it freaked out, and said forget it!!   

The default email address was set to ******>>> in this case, which is clearly an email address using "illegal characters", so the sending process was aborted.