When someone wants to send you a secure message (encrypted), He/She needs your public key.
If He/She has not your public key yet, he can search the keyserver for your email or your name and get your public key that way.

When you digitally sign a message the recipient of the message needs your public key in order to verify the signature.

The quick answer is: you cannot. The relevance and usability of a key is based solely on its expired and/or revoked status.

This keyserver is not alone. There are a lot of computers in the world keeping the public keys of everyone.
Those public keyservers are exchanging their keys on a regular basis.
This means that when a keyserver gets a key, it is a matter of days until all the keyserver have a copy of it.
Even if a keyserver deletes your key, he will get it back from another keyserver sooner or later.

We got so many requests of this nature that we have devised a solution.
You cannot truly remove a key, but instead you can ask to the Administrator of the keyserver to hide your key.
Your key will no longer be shown in any search request on this keyserver.
Basically, your key will no longer exist for any user of www.keyserver.net.
This is not a perfect solution. Your key will still be visible for the users looking through the other keyservers.