If you have vim >= 7 (I think) you can use autocmd with an event SwapExists. There you can do all from very simple things to very complicated.
Here is a simple example to put in your ~/.vimrc:
autocmd SwapExists * let v:swapchoice = "o"
If a swap file exists, this event will be triggered. The autocmd above will simply open the file in read only mode.
If v:swapchoice gets a value in an autocmd it will not prompt you what you want to do. The values you can use is listed here v:swapchoice.
For a very advanced autocmd see here: editexisting.vim
The script there can already be on your server from the installation of vim, search for editexisting.vim.
Best Answer
You could do this:
That unsets the read-only flag, but if the underlying file is still not writable by you then vim still will be unable to write to it.