Disabling illegal coverpoint check?

You can use the built-in covergroup methods to disable sampling.

mycovergroup group1 = new();

initial

begin

   ...

   group1.stop();

   ...

   group1.start();

endgroup

You could also use a global signal and the "iff" clause in a covergroup declaration to control more than one covergroup at a time.

reg illegal_packet_ok = 0;

covergoup moncover @(posedge clk);

   coverpoint mon_state iff ( ! illegal_packet_ok ) ...

There are a lot of places in the covergroup syntax where you can place the iff-clause to turn off a portion of the covergroup, a portion of a coverpoint or a portion of a cross. So you can experiment to see exactly where and when you want to disable the collection of data for an illegal packet.