You have choices!
You can enable a databaseless scanning mode by editing /etc/mail/canit/canit.conf on all your scanners and adding these lines:
[filter] database_down_action = minimalfilter database_down_tag_score = 5 database_down_reject_score = 20
This will run a minimal tag-only SpamAssassin filter while the database is down, tagging at 5 points and rejecting at 20.
If you would prefer to have messages delivered <shudder> unscanned in this case, add or edit the database_down_action configuration option to contain accept (for example, database_down_action = accept). In this case, all mail will have a header like this added:
X-Scan-Warning: NOT SCANNED BY CANIT
You need to restart CanIt on each scanner for the changes to the file to take effect in either case.