According to the <font color="orange">ifconfig man page</font>:
To delete a logical interface, simply name the interface specifying an address of 0, after ensuring that the inter- face has been marked as "down". For example, the command:
ifconfig le0:1 0 down will delete the logical interface le0:1.
The previous comment is correct that unplumbing a logical interface in Solaris will unplumb the physical interface, as plumbing is a "physical" matter.