Can it be turned off or changed to a different value?

> DataFrame(test = paste(LETTERS, collapse = ''))
DataFrame with 1 row and 1 column
                    test
             <character>
1 ABCDEFGHIJKLMNOPQRST..

?`show,DataFrame-method` seems not.

As a DataFrame? No. But if you want conventional data.frame behavior, you can just convert.

> as(DataFrame(test = paste(letters, collapse = "")), "data.frame")
                        test
1 abcdefghijklmnopqrstuvwxyz