Vim run autocmd on all filetypes EXCEPT

*.rb isn't a filetype. It's a file pattern. ruby is the filetype and could even be set on files that don't have a .rb extension. So, what you most likely want is a function that your autocmd calls to both check for filetypes which shouldn't be acted on and strips the whitespace.

fun! StripTrailingWhitespace()
    " Don't strip on these filetypes
    if &ft =~ 'ruby\|javascript\|perl'
        return
    endif
    %s/\s\+$//e
endfun

autocmd BufWritePre * call StripTrailingWhitespace()

Building on evan's answer, you could check for a buffer-local variable and determine whether to do the strip using that. This would also allow you to do one-off disabling if you decided that you don't want to strip a buffer that's a filetype you normally would strip.

fun! StripTrailingWhitespace()
    " Only strip if the b:noStripeWhitespace variable isn't set
    if exists('b:noStripWhitespace')
        return
    endif
    %s/\s\+$//e
endfun

autocmd BufWritePre * call StripTrailingWhitespace()
autocmd FileType ruby,javascript,perl let b:noStripWhitespace=1

Another choice of one line way:

let blacklist = ['rb', 'js', 'pl']
autocmd BufWritePre * if index(blacklist, &ft) < 0 | do somthing you like

Then you can do something you like for all filetypes except those in blacklist.

A good way would be to set a local variable for the one filetype to true. Then set the automcommand if that variable is false (if set for everything else) or if it exists at all (no need to preset it).

autocmd BufWritePre *.foo let b:foo=true

if !exists("b:foo")
    autocmd ...
endif

changed variable prefixes based on comment

You can do the except on the same regexp:

autocmd BufWritePre *\(.out\|.diffs\)\@<! <your_command>

That will do <your_command> for all files extensions except for .out or .diffs.