ASM - Block GET requests on a specific URL

1 - Creating a new User-defined ASM violation

Security > Options > Application Security > Advanced Configuration > Violations List > User Defined Violations (tab); Select "Create New User-Defined Violation"

Sample Field Values (adjust as needed):
Name "VIOLATION_FORBIDDEN_GET_PATH"
Title "GET Request to a restricted path"
Type "Access Violation"
Severity "Alert"
Attack Type "Abuse of Functionality"
Description (leave empty)

2 - Go to Blocking Settings

Select Block for your new custom violation (or Alarm, if you want to transparently test)

3 - Creating an iRule

The sample below covers the most simple use-case, a single path. In case of 10 or more paths, using a LTM data group entry match, or a switch statement would be a better option.

when HTTP_REQUEST {
  set reqBlock 0
  if {([HTTP::method] equals "GET") and ([string tolower [HTTP::path]] equals "/mypath/index.aspx")}{
    set reqBlock 1
  }
}
when ASM_REQUEST_DONE {

  if { $reqBlock == 1} {
    ASM::raise VIOLATION_FORBIDDEN_GET_PATH
  }
}