We can and we do to some extent. The problem is that on the high seas it is the country of registration that you need permission from to board. It wouldn't cost much to offer the poor countries that register the worst offenders to either stop registrations or be more permissive with boarding rights.
It doesn't even have to be governments, Sea Shepherd do some good work and show what can be achieved.
The other problem is poor countries not able to secure their own territorial waters and Exclusive Economic Zones. For instance in West Africa many navies and coastguards have ships with endurance measured in days. If you're up to no good you just wait for them to come alongside. RUSI are currently floating the idea of giving them proper offshore patrol vessels paid for from the aid budget to enable them to actually patrol and secure their waters.
