If you aren't wanting to affect the ability of littlegroup to use all commands they have access to, you wouldn't use can_target.
Can_target is group specific, not command specific. Though you might prevent littlegroup from using ulx bump, you'd also prevent them from using any other ulx commands on any other group.
To adjust access to commands specifically, you could use something like ' ulx groupallow littlegroup "ulx bump" "#littlegroup" '