You can delete a variable by right-clicking its name and selecting Cut from the context menu.
Once you have selected a variable you can delete it by clicking on the icon on the main toolbar or by pressing Del.
If you have selected multiple variables, they will all be deleted.
Caution:
If any rules' condition or action parts refer to the variable you have removed, the references will be reverted back to their non-terminals. This will make those rules, and therefore the enclosing states and the scenario, unfinished.