I employed the services of a php programmer to create a routine to remove users from roundcube 'completely'.
This routine also verifies if (perhaps) someone has tried to remove a user directly thru phpMyAdmin, and has created orphaned records.
You also can delete any specific roundcube user and all related entries easily and cleanly.
a tarball has been placed on my ftp site. This has been tested with both 0.5.1 & 0.5.4... MySQL only.
While I 'paid' the programmer for this work, I offer the source to the roundcube community as my contribution to the furtherance of roundcube.
ftp://ftp.haidagwaii.net/roundcube/