Hi Bsiki,
turning it off is not possible, a far as I can tell. However, you can customize it, i.e. replace it with HTML of your own. So for example you can create an empty page, name it empty.html, then import it in ASDM -> configuration -> remote access -> Clientless SSL VPN access -> portal -> help customization.
see also
http://www.cisco.com/en/US/products/ps6120/products_tech_note09186a008094abcb.shtml#helpapps
The result is not as good as if you would be able to turn it off, but I hope this still helps.
If this is a strong requirement you could also open a TAC case and ask that an enhancement request be submitted, or (even better) contact your account team and ask them to submit a product enhancement request.
hth
Herbert
