You don't need a regular expression for that. I would do

string = "What $nounverb$?"
print string.replace("$noun$", "the heck")

Only use regular expressions when needed. It's generally slower.

You can do that with a single call to sub:

big_regex = re.compile('|'.join(map(re.escape, prohibitedWords)))
the_message = big_regex.sub("repl-string", str(word[1]))

Example:

>>> import re
>>> prohibitedWords = ['Some', 'Random', 'Words']
>>> big_regex = re.compile('|'.join(map(re.escape, prohibitedWords)))
>>> the_message = big_regex.sub("<replaced>", 'this message contains Some really Random Words')
>>> the_message
'this message contains <replaced> really <replaced> <replaced>'

Note that using str.replace may lead to subtle bugs:

>>> words = ['random', 'words']
>>> text = 'a sample message with random words'
>>> for word in words:
...     text = text.replace(word, 'swords')
... 
>>> text
'a sample message with sswords swords'

while using re.sub gives the correct result:

>>> big_regex = re.compile('|'.join(map(re.escape, words)))
>>> big_regex.sub("swords", 'a sample message with random words')
'a sample message with swords swords'

As thg435 points out, if you want to replace words and not every substring you can add the word boundaries to the regex:

big_regex = re.compile(r'\b%s\b' % r'\b|\b'.join(map(re.escape, words)))

this would replace 'random' in 'random words' but not in 'pseudorandom words'.