Here is the scenario: I have an Icy Manipulator and opponent has Llanowar Elf. If I tap the elf with icy, can he respond by tapping it for mana? Conversely, if he taps for mana can I respond by tapping with icy to prevent the mana?
While the maipulator ability is on the stack to tap your elf, the elf can tap to put his "add 1 g to your mana pool" over top of icy manipulator's ability. The elf ability will resolve adding 1 g, when manipulators ability attempts to resolve its legal target will no longer be legal and it will 'fizzle.' if the play starts with the tapping of the elf their response with manipulator is again futile because tapping the elf is a cost of playing the ability. You cannot counter someone from paying to activate an ability, but you can counter abilities. As far as countering a mana ability, it cannot be done... Read {Stifle}... But you could {Stifle} a tapped and sacrificed {Misty Rainforest}. This would result in them sacking their land to get nothing, because the fetch ability is countered.
This is all correct. Mana abilities are tricky because you can use them like other abilities and place them on the stack, but usually you activate them while paying for a spell/activation, in which case they are used in the middle of casting a spell and don't use the stack.
Basically, in this case, if you want to hose their {Llanowar Elf}, make them tap it at a bad time, like upkeep or combat phases. That way, there are fewer things they could do with the mana, even if they do get to tap it anyway. ;)
What Blackjester suggested is the best you can do in this situation. Mana is emptied at the end of each phase, so making your opponent tap him during upkeep denies him the mana during Main Phase.
What about Rishadin Port? Can you effectively tap another player's land with it, thus rejecting them the mana it could produce? Or is the best you can do is make them use the mana during say their upkeep?
The second ability of the {Rishadan Port} would work the same way the {Icy Manipulator} does in this example. You can't get around them putting the mana in their pool. You can only force them to add it at inconvenient times.
If you force them to pay in their upkeep does the mana stay in the mana pool until the end of their turn? Or does mana empty at the end of each phase?
It empties at the end of each phase, as stated earlier.
Actually, now mana pools empty at the end of each phase and each step too.
Last question: can I respond to (Gideon's Lawkeeper) ability by tapping the Lawkeeper with (Icy Manipulator) and prevent the Lawkeeper from tapping my creature?
Nope.
Since they have already declared the ability, and tapping the {Gideon's Lawkeeper} is part of the cost, by the time you get priority to do anything, he's already tapped and the ability is on the stack.
Ok ok and if I tap it with the icy first can he respond by using the Lawkeeper ability?
You got it! Sucks, eh? :)
Yup....thanks for clarifying