I think you are mixing two formats (VBscript and Python). The field SITE using the Python parser should be expressed as !SITE!.
You could use something like this with the Python parser:
def FindLabel(site):
return "" if str(site).startswith('18') == True else site
Your expression should be:
Findlabel(!SITE!)
I tested it with this:
def main():
tests = [18, 182, 1807, 18874, 1901, 17, 1981]
for test in tests:
print "in: {}, label:{}".format(test, FindLabel(test))
def FindLabel(site):
return "" if str(site).startswith('18') == True else site
if __name__ == '__main__':
main()
which resulted in:
in: 18, label:
in: 182, label:
in: 1807, label:
in: 18874, label:
in: 1901, label:1901
in: 17, label:17
in: 1981, label:1981
All values that start with 18 result in an empty string