Technically, trade in currency could be constrewed as foreign trade and is subject to tarrifs, not taxes. That aside, taxes have typically been used to discourage trade and bolster the local national market. If it is cheaper to buy local, any good capitalist will. Trading currency is the same principle. If there is a penalty already involved, then most people will not take the risk of trading currency.
Tobin taxes are taxes levied on currency exchange transactions in order to hold traders accountable for exploiting "noise" or high volume when trading currencies across borders internationally. Tobin taxes add yet another layer of market regulation and would likely do little to reduce market volatility or increase stability. As a student of Keynes, Tobin's economic thinking is along the lines of "If it ain't broke, DO fix it." Currency exchange is a thriving financial sector which encourages global investment and should not be hampered by increased taxes or regulations.
Yes, I think that Tobin taxes in electronic currency exchanges discourage currency trading, because they they make it more expensive to trade currency. Any time an activity is going to be made more expensive, there are fewer people who will engage in that activity. This is no different than any other increase in price.
No, I do not think that these taxes discourage currency trading very much at all. I think that if you need to use this machine, you will not worry all that much about a small tax that is being charged to you so that the government gets helped out alot.
The reason I don't think that Tobin taxes in electronic currency exchanges discourages currency trading is because there hasn't been any evidence of anything discouraging currency exchanges. Tobin taxes are a hindrance and annoyance, of course, but they are certainly not going to deter or discourage currency trading on any level.