You can use the following.
var str = '@foo @bar @baz RT @somename: status',
res = str.replace(/RT\s*@\S+/g, '');
console.log(res); // => "@foo @bar @baz status"
Regular expression:
RT 'RT'
\s* whitespace (\n, \r, \t, \f, and " ") (0 or more times)
@ '@'
\S+ non-whitespace (all but \n, \r, \t, \f, and " ") (1 or more times)
Another option would be just to match up until and include the colon.
str.replace(/RT\s*@[^:]*:/g, '');