I believe you can kill the file /home/httpd/html/horde/imp/config/ldap.php3 will do the trick. You will need to do this via a template else the system will rebuild the file later.
I have not tested this, so be careful, but I think it is safe to do so.
Probably a better way is that you can refine the use of LDAP (as well as disable it all together) in several settings in defaults.php3 in the same directory as above. It should be pretty self-explanatory, the file is well documented. Once again, use the templates.
Hope this helps.
JP