For browsers that support it, there's the new autocomplete attribute. From the link:
The autocomplete attribute is an enumerated attribute. The attribute has three states. The on keyword maps to the on state, and the off keyword maps to the off state. The attribute may also be omitted. The missing value default is the default state.
The off state indicates either that the control's input data is particularly sensitive (for example the activation code for a nuclear weapon); or that it is a value that will never be reused (for example a one-time-key for a bank login) and the user will therefore have to explicitly enter the data each time, instead of being able to rely on the UA to prefill the value for him; or that the document provides its own autocomplete mechanism and does not want the user agent to provide autocompletion values.
...although in the specific case you describe, I think I'd probably just keep the € symbol outside the field's value in the first place. You can put it next to the field, or superimpose it on the field using CSS (for instance, put it after the field in the markup and then use position: relative; left: -3em, that sort of thing). But if you really want to prevent the browser's auto-filling, autocomplete is one tool for the toolchest.
