It don't matter, any way of hiding that from anything is a violation of your licensing agreement. Be it a search engine, bot, spider, ect.

All copyright messages (both in the software code and templates) must be retained and visible.

A 'branding free' license is available at a cost of $170 per license, in addition to the cost of the license itself.

Once you have purchased this option, you will have the right to remove all references to vBulletin from the vBulletin design templates. Note this is for one license only and is in addition to the cost of the license.

Also please note that you must retain all copyright notices within the actual product programming code (these notices do not affect any visible vBulletin pages). Note also this does not allow you to rebrand the software with another name. It only allows you to remove the vBulletin copyright information.

The vBulletin branding-free license concerns the removal of visual branding. The branding-free package available from the members' area does not remove comments or variable names within the program code that mention vBulletin.

Thanks for the information ;)
Okay I will not remove the vbulletin copyright.

I also want remove my ads from search engine spider, I don't spider can read my my ads. So I still need that code. Can you help me, please? Many thanks.

Kind regards,
Shine

Only way to do that is not to display the ads to guests.

Also this will not only get you blacklisted from search engines, it will also get you banned from google adsense.

You can change that phrase, it's no big deal at all. On my boards it typically goes:

It's true the script kiddies google up vB version numbers by "powered by" to find one that isn't updated or is still vulnerable to some exploits.

Spam however is a different story and hiding your site from search engines won't inhibit it in any way.

Then that is a clear violation of the license agreement, at a minimum it must say:
"Powered by vBulletin - Copyright ? vBulletin Solutions, Inc. All rights reserved."

Meaning you can remove the link, and the version number.