Rishi,
when s.o. adds a user to a project the role-selection-screen pops up (that's
where you attribute a role to the user). Disable the ok button on this
screen using the security workbench. The user will have to end the screen
with Cancel, which cancels out the whole process and the user will not be
added to the project.
Do not disallow the screen totally. The user would get the usual security
message, but would still be included to the project (at least it used to be
some time ago, kind of a bug).
This also helps preventing anyone off changing his/her role.
Good luck, Gerd
B732 - ERP8, Unix, Windows, Oracle, SQL, WTS, JAS