I already searched in the commands ingame via help and tried to find a file containing all portals but with no luck.
If its easy to remove portals and Im just overlooking it, it would be good to add this information to the /help portal command.
Solutions from the replies:
For non solid portals (air/water/lava) you can just place a block in it so the portal disappears or use /z air to remove it.
For solid portals (blue/orange) you can use /delete and just break the blocks or /z air to replace them with air which also removes the portal.