Inno Setup installers don't, by default, start an app after installing. (They can optionally be written to do so, of course, but this is at the discretion of the installer's author.)
If the installer you are running has "hard-coded" the application to start automatically after installation, then no, there is no command line parameter that would prevent that (unless the installer's author built the installer to support it; see below).
Some potential exceptions to this (again, depending on how the installer is designed) are:
* If the installer is using an IS task to decide whether to start the application, then you would omit that task when installing
* If the installer supports a custom command-line parameter to prevent launching the app, then use that (the only way you would know this parameter exists would be to examine the installer source or ask the author)
These types of exceptions would depend on how the installer is built.
If these exceptions don't apply to the installer you're running, then I would say that the author of the installer is not doing things in a very polite way or possibly hasn't considered that someone might want to install silently.
Bill