If you recently upgraded to SpamAssassin 3.x, URIBL was included by default. It can be disabled by adding the following lines to your spamassassin/local.cf:
score URIBL_BLACK 0 score URIBL_RED 0 score URIBL_GREY 0 score URIBL_BLOCKED 0
To disable the lookups, we normally suggest you put the above lines into the file /etc/mail/spamassassin/99_local.cf, as since neither we nor SpamAssassin ship a file by that name, it survives upgrades.
You'll need to put those lines on all the scanners; they become effective when the command
is given on the command-line.