iRule to drop if uri starts with specific word.

You can certainly use "contains" instead of "starts_with" but will it achieve the functionality that you are seeking ? I think you need to discuss with the customer to understand the function required and also inform him of what exactly your iRule will do before implementing.

I would recommend logging the actual URI that is being dropped to compare:

Using Joe's iRule:

when HTTP_REQUEST {
  if { [string tolower [HTTP::uri]] starts_with "/abc/" } {
    log local0. "Rejecting [HTTP::uri] request"
    reject
  }
}