You can trademark a word, logo, phrase, design, or combination of these things. If you only TM the word other companies can still use it as long as it won't confuse the curent customer base; in other words it would have to be a different type of business or if it is a similar business type you must TM the words as a logotype so as not to confuse customers for the other business.
That said, if we drasitcally change what Avengers is and make a different styalized logotype for it we can use it. Most people just avoid using anything which might cause undue stress, legal disputes and negative press though.