Due to the template engine I am using, I cannot use IF/ELSE statement.

==============================
<form>

At the moment you use a regular expression approach.
You can keep that, but first check for keyvalues CARRIAGE RETURN and
BACKSPACE, and return true if one of them.

So you'll end up with something like:

<textarea name="event_description" ONKEYPRESS="if (document.layers)
                      var c = event.which;
                    else if (document.all)
                      var c = event.keyCode;
                    else
                      var c = event.charCode;
                    var s = String.fromCharCode(c);
                    if (s == 13) {return true;}
                    if (s == 32) {return true;}
                    return /[0-9a-zA-Z\s,.?!@#$%&*()-]/.test(s);"></

The 13 is ENTER
The 32 is space, not BACKSPACE.
I don't remember the number of BACKSPACE. You'll have to find the number
yourself. (Or use alert(s) to test).