Harid,
The .tri file can be deleted using the /fdele command if it is not required in solution (as long as this command is submitted at the "begin" level early on). Also, if it is required in solution, it will only delete this file AFTER the solution has finished. There is a /delete option, which will delete the file regardless (dangerous), but you have to ask yourself WHY the file is not being deleted in the first place - most probably because it is required during solution! If you are constrained by your PC capability, the only thing you can do is reduce your model size unfortunately.
Best of luck!
-- drej --