...so if they don't need to alter the artwork except scale it to a given dimension then a correctly produced PDF file that can be scaled would suffice...
...restrictions can be applied to PDF files upon saving from illustrator, but an ai file itself cannot be restricted, there used to be a plugin for illustrator 8 but no idea if it is still around, but even that was easy to get around by saving to eps...
...even pdf files can be stripped of security for those that have the software to do it...
...in any case, i can't imagine why your print provider would need to alter your files in anyway even if you supply them the ai files > fonts > images, unless you supply inadequate artwork that is problematic to scale...
...and to save any possible font problems you ought to outline those, or supply a subset font embedded pdf file...
Andrew