Hi,
You cant delete it inside Acrobat security settings as this requires the use of a password, however, if you can find the actual ID file on the computer, you can remove it from here.
On a mac, do a serch for .p12 and you will find it, on the PC look for a .pfx file.
As default they usually go in the 'security' folder nested inside your applications settings. On a mac its library-application support - Adobe – Acrobat – (Acrobat version number here) - security – digital ids. Its similar on a PC, i thing you should start of with 'C:Program files'
Hope this helps?