Can you travel 'above' the control of a country?

A convention sort of exists, in that countries recognize there's a limit, but they don't exactly agree on what the limit is. In any case, though the average consumer isn't getting that high up (at least for now).

Wikipedia says, on the subject of how high sovereignty extends,

There is no international agreement on the vertical extent of sovereign airspace (the boundary between outer space—which is not subject to national jurisdiction—and national airspace), with suggestions ranging from about 30 km (19 mi) (the extent of the highest aircraft and balloons) to about 160 km (99 mi) (the lowest extent of short-term stable orbits).

19 mi is roughly twice the largest absolute ceiling of commercial and business aircraft; you might be able to get up there with a military jet, but well, if you're flying military, the rules are probably bendable anyway.

I would like to add to the other answers that once you pass the 30km (19 mi) height (which is possible with a stratospheric balloon) you are effectively outside of the actual control of a country. There is no weapon or force available in this height which can reach you. Most of the available missile systems reaches up to 25 km and even the SR-71 Blackbird cannot fly higher than 26 km horizontally. The US and Russia may have anti-satellite weapons, but any other country is pretty helpless against people flying in this height.

While there is no international legal agreement as to a specific altitude at which space begins, international law defines the lower boundary of space as the lowest perigee attainable by an orbiting space vehicle. The term "orbiting" implies unpowered flight, which given atmospheric drag means an altitude of around 150 kilometres.

Alternatively, the Karman line at 100 kilometres altitude is accepted by the Fédération Aéronautique Internationale as the boundary between airspace and outer space. It is at this altitude that the atmosphere becomes too thin to support aeronautical flight (using wings to generate lift) as the speed required to generate lift is greater than the speed of a ballistic trajectory (rocket flight).