Melira’s Keepers has a static ability that prevents counters from being placed on it. That does not mean that it cannot be targeted by spells or abilities that would attempt to put counters on it. For example, Melira’s Keepers can be targeted by the activated ability of Vedalken Anatomist. When the Anatomist’s ability resolves, the -1/-1 counter will not be placed on the Melira’s Keepers, but the controller of the Anatomist’s ability can choose to tap or untap the Keepers.