I'm currently playing on a Hard Mode Minecraft server. About a week ago I hit a zombie pigman, and since then they have been aggressive no matter where I go in the nether. I have even gotten on top of the Nether and run as far as possible, but they're still aggressive when I return.
Notable information from the wiki on the topic of Zombie pigmen:
Zombie pigmen will remain hostile for 20 to 39.95 seconds, and will continue to pursue an already-targeted player after this time until the player escapes their 40 block pursuit radius.
If you're getting farther than 40 blocks away from the nearest pigman, and haven't provoked them within the last 39.95 seconds, you should be ok. My suspicion would be that you have not stayed close enough to keep the chunks the angered pigmen are in loaded, since:
The forgiveness timer does not advance for zombie pigmen in unloaded chunks. Thus, if the player enters a Nether portal and returns, any provoked zombie pigmen in the area will still be aggressive unless another player was nearby to keep the chunks loaded.
You need to be far enough away that the pigmen stop tracking you, but not so far away that the chunks they are in unload.