what can glaxosmithkline do to improve their image with Americans and
stop Internet sales from Canada to the United States?

One very simple thing: lower their prices.

To add to my very quick comment:

Americans buy prescription drugs from Canadian online pharmacies for
the very simple reason that the drugs are cheaper. Period.

GSK can lower their American prices to eliminate this differential.
This would be better than what they are currently doing, which is
lobbying the US government for increased regulation and trade
restrictions (they are doing this along with every other big pharma...
net result of increased regulation is increased cost to the consumer).
It will also be better than the current big pharma tactic of hinting
that they will restrict the supply of drugs to Canada to match what
they project are the national requirements, implying (without actually
stating) that Internet drug sales will deprive Canadians of the drugs
they need themselves.