Forum OpenACS Improvement Proposals (TIPs): Re: TIP #14 (Proposed): Making templates safe by removing tcl tag.
The preprocessor for the templating system replaces <% with <tcl>, so you just need to disable the <tcl> tag in the templatng system.
As others have pointed out, some packages still rely on this tag and inline tcl, so you could never, and probably should never, remove it completely. An individual install could easily disable it anyway.
One additional note: the ATS is very good a separating application logic from display logic. I would say it is one of the best systems available, if not the best. Removal of the tcl tag may make it safe to allow users to upload their own templates for pages.