Zersetzung.

From Wikipedia:

The Stasi perfected the technique of psychological harassment of perceived enemies known as Zersetzung – a term borrowed from chemistry which literally means "corrosion" or "undermining".

By the 1970s, the Stasi had decided that methods of overt persecution which had been employed up to that time, such as arrest and torture, were too crude and obvious. It was realised that psychological harassment was far less likely to be recognised for what it was, so its victims, and their supporters, were less likely to be provoked into active resistance, given that they would often not be aware of the source of their problems, or even its exact nature. Zersetzung was designed to side-track and "switch off" perceived enemies so that they would lose the will to continue any "inappropriate" activities.

Tactics employed under Zersetzung generally involved the disruption of the victim’s private or family life. This often included breaking into homes and messing with the contents – moving furniture, altering the timing of an alarm, removing pictures from walls or replacing one variety of tea with another. Other practices included smear campaigns, denunciation, provocation, psychological warfare, psychological subversion, wiretapping, bugging, mysterious phone calls or unnecessary deliveries, even including sending a vibrator to a target's wife. Usually victims had no idea the Stasi were responsible. Many thought they were losing their minds, and mental breakdowns and suicide could result.

One great advantage of the harassment perpetrated under Zersetzung was that its subtle nature meant that it was able to be denied. That was important given that the GDR was trying to improve its international standing during the 1970s and 80s. Zersetzung techniques have since been adopted by other security agencies, particularly the Russian Federal Security Service (FSB).