You will have to use the MPF feature on the ASA to block the GoToMyPC traffic.
Configure the policy-map type like this
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policy-map type inspect http GotoMyPC_HTTP
parameters
match request uri regex _default_GoToMyPC-tunnel "machinekey"
drop-connection log
match request uri regex _default_GoToMyPC-tunnel_2 "[/\\]erc[/\\]Poll"
drop-connection log
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Also refer to this document
http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a00808c38a6.shtml
Hope this helps.