Use the uninstaller to remove Instant JChem from your machine.
There are a couple of ways to remove Instant JChem:
Double click on uninstall.exe in the Instant JChem’s home folder
Select Instant JChem from the Add / Remove programs list on Control Panel.
Select “Instant JChem’s uninstaller” from the “ChemAxon/Instant JChem” menu from the Programs menu (which you can access by pressing the “Start” button on the toolbar).
Drag the icon of Instant JChem into Trash.
Linux / Solaris
Launch the uninstall script in the Instant JChem’s home directory.