Naban's ability works only on the battlefield (since it can work there and doesn't mention another zone where it works) (C.R. 117.2c, 611.3b). Thus if Naban leaves the battlefield, Wizards entering the battlefield under your control don't cause certain abilities to trigger more often than otherwise. More concretely, if you cast a Wizard creature spell and Naban leaves the battlefield before that spell resolves, that Wizard won't benefit from Naban's effect upon entering the battlefield.
Merfolk Trickster's triggered ability doesn't counter any abilities on the stack, even if they come from the targeted creature (C.R. 113.7a; C.R. 113.1a-c explain what abilities are). For instance, if the creature is Goblin Sharpshooter, the Merfolk Trickster ability will remove Goblin Sharpshooter's abilities until end of turn, including its last ability ("T: Goblin Sharpshooter deals 1 damage to any target" [C.R. 108.1]), but it doesn't counter any abilities on the stack from Goblin Sharpshooter that have already triggered or been activated, so that those abilities will still get to resolve as normal (C.R. 113.7a).
EDIT (Oct. 4): Edited to conform to rule changes with Guilds of Ravnica.
EDIT (Feb. 27, 2021): Some rules were renumbered in the meantime.
As for Trickster, if a Monster (0/0), that has the ability to gain X amount of power passively, attacks and I flash with Trickster and Block with Trickster, that monster dies?
As for Trickster, if a Monster (0/0), that has the ability to gain X amount of power passively, attacks and I flash with Trickster and Block with Trickster, that monster dies?
If you're talking about a creature (not: monster) like Maro that sets the creature's power and toughness, then removing all that creature's abilities with Merfolk Trickster's triggered ability will, in general, result in the creature no longer having a defined power and toughness, and instead having power and toughness 0/0 (C.R. 107.2, 613.1f-g; see also C.R. 208.5). Then, after the ability resolves, that creature will go to the graveyard if its toughness is 0 or less — this happens before a player gets priority, and well before you get a chance to declare blockers in this scenario (C.R. 117.3b, 117.5, 704.5f, 508.2). See also this thread and this thread.
However, if you're talking about a creature like Copperhoof Vorrac or Shanna, Sisay's Legacy, then removing that creature's abilities, in general, simply means that power and toughness won't be added to by its abilities (review C.R. 613.4c). For example, Copperhoof Vorrac would then not get +1/+1 for each untapped permanent "your opponents control", nor would Shanna get +1/+1 for each creature "you control". Those two creatures would then be 2/2 and 0/0, respectively, if nothing else is affecting their power and toughness.
EDIT (Mar. 23, 2019): Edited.
EDIT (Feb. 27, 2021): Edited, including because some rules were renumbered in the meantime.
Also, does merfolk trickster negate effects in response to someone tapping their abilities?
Merfolk Trickster's triggered ability doesn't counter any abilities on the stack, even if they come from the targeted creature (C.R. 113.7a; C.R. 113.1a-c explain what abilities are). For instance, if the creature is Goblin Sharpshooter, the Merfolk Trickster ability will remove Goblin Sharpshooter's abilities until end of turn, including its last ability ("T: Goblin Sharpshooter deals 1 damage to any target" [C.R. 108.1]), but it doesn't counter any abilities on the stack from Goblin Sharpshooter that have already triggered or been activated, so that those abilities will still get to resolve as normal (C.R. 113.7a).
EDIT (Oct. 4): Edited to conform to rule changes with Guilds of Ravnica.
EDIT (Feb. 27, 2021): Some rules were renumbered in the meantime.
As for Trickster, if a Monster (0/0), that has the ability to gain X amount of power passively, attacks and I flash with Trickster and Block with Trickster, that monster dies?
However, if you're talking about a creature like Copperhoof Vorrac or Shanna, Sisay's Legacy, then removing that creature's abilities, in general, simply means that power and toughness won't be added to by its abilities (review C.R. 613.4c). For example, Copperhoof Vorrac would then not get +1/+1 for each untapped permanent "your opponents control", nor would Shanna get +1/+1 for each creature "you control". Those two creatures would then be 2/2 and 0/0, respectively, if nothing else is affecting their power and toughness.
EDIT (Mar. 23, 2019): Edited.
EDIT (Feb. 27, 2021): Edited, including because some rules were renumbered in the meantime.