That depends on what you mean. If you just want to get rid of them, do this:
(Update: Apparently you want to keep digits as well, use the second lines in that case)

String alphaOnly = input.replaceAll("[^a-zA-Z]+","");
String alphaAndDigits = input.replaceAll("[^a-zA-Z0-9]+","");

or the equivalent:

String alphaOnly = input.replaceAll("[^\\p{Alpha}]+","");
String alphaAndDigits = input.replaceAll("[^\\p{Alpha}\\p{Digit}]+","");

(All of these can be significantly improved by precompiling the regex pattern and storing it in a constant)

Or, with Guava:

private static final CharMatcher ALNUM =
  CharMatcher.inRange('a', 'z').or(CharMatcher.inRange('A', 'Z'))
  .or(CharMatcher.inRange('0', '9')).precomputed();
// ...
String alphaAndDigits = ALNUM.retainFrom(input);

But if you want to turn accented characters into something sensible that's still ascii, look at these questions:

• Converting Java String to ASCII
• Java change áéőűú to aeouu
• ń ǹ ň ñ ṅ ņ ṇ ṋ ṉ ̈ ɲ ƞ ᶇ ɳ ȵ --> n or Remove diacritical marks from unicode chars

Simply use String#replace(CharSequence target, CharSequence replacement) in your case to replace a given CharSequence, as next:

special = special.replace("@$", "as");

Or use Pattern.quote(String s) to convert your String as a literal pattern String, as next:

special = special.replaceAll(Pattern.quote("@$"), "as");

If you intend to do it very frequently, consider reusing the corresponding Pattern instance (the class Pattern is thread-safe which means that you can share instances of this class) to avoid compiling your regular expression at each call which has a price in term of performances.

So your code could be:

private static final Pattern PATTERN = Pattern.compile("@$", Pattern.LITERAL);
...
special = PATTERN.matcher(special).replaceAll("as");

You can go the other way round. Replace everything that is not word characters, using negated character class:

message = message.replaceAll("[^\\w]", "");

or

message = message.replaceAll("\\W", "");

Both of them will replace the characters apart from [a-zA-Z0-9_]. If you want to replace the underscore too, then use:

[\\W_]

That depends on what you define as special characters, but try replaceAll(...):

String result = yourString.replaceAll("[-+.^:,]","");

Note that the ^ character must not be the first one in the list, since you'd then either have to escape it or it would mean "any but these characters".

Another note: the - character needs to be the first or last one on the list, otherwise you'd have to escape it or it would define a range ( e.g. :-, would mean "all characters in the range : to ,).

So, in order to keep consistency and not depend on character positioning, you might want to escape all those characters that have a special meaning in regular expressions (the following list is not complete, so be aware of other characters like (, {, $ etc.):

String result = yourString.replaceAll("[\\-\\+\\.\\^:,]","");

If you want to get rid of all punctuation and symbols, try this regex: \p{P}\p{S} (keep in mind that in Java strings you'd have to escape back slashes: "\\p{P}\\p{S}").

A third way could be something like this, if you can exactly define what should be left in your string:

String  result = yourString.replaceAll("[^\\w\\s]","");

This means: replace everything that is not a word character (a-z in any case, 0-9 or _) or whitespace.

Edit: please note that there are a couple of other patterns that might prove helpful. However, I can't explain them all, so have a look at the reference section of regular-expressions.info.

Here's less restrictive alternative to the "define allowed characters" approach, as suggested by Ray:

String  result = yourString.replaceAll("[^\\p{L}\\p{Z}]","");

The regex matches everything that is not a letter in any language and not a separator (whitespace, linebreak etc.). Note that you can't use [\P{L}\P{Z}] (upper case P means not having that property), since that would mean "everything that is not a letter or not whitespace", which almost matches everything, since letters are not whitespace and vice versa.

Additional information on Unicode

Some unicode characters seem to cause problems due to different possible ways to encode them (as a single code point or a combination of code points). Please refer to regular-expressions.info for more information.