If I understand your question, you want to limit someones ability to "do something", for example; activate a button, slider or switch (in the app) while the phone or tablet is in a specific area?
If so, then I belive you can use the GPS trigger, attached to a virtual pin, that then acts like a flag for another programmed function.
Perhaps like this:
BLYNK_WRITE(V0) // GPS Trigger
GPSflag = param.asInt(); // Set the flag to 1 or 0
BLYNK_WRITE(V1) // Button that does something if GPSflag is 1
if (GPSflag == 1) // Check flag
// Do something if flag is 1
// Do something else and/or give feedback that something is not allowed in this location