R not responding request to interrupt stop process
Unfortunately, RStudio is currently not able to interrupt R in a couple situations:
R is executing an external program (e.g. you cannot interrupt
system("sleep 10")),R is executing (for example) a C / C++ library call that doesn't provide R an opportunity to check for interrupts.
In such a case, the only option is to forcefully kill the R process -- hopefully this is something that could change in a future iteration of RStudio.
EDIT: RStudio v1.2 should now better handle interrupts in many of these contexts.
This could happen when R is not working within R and is invoking an external library call. The only option is to close the project window. Fortunately, unsaved changes including objects are retained on opening RStudio again.