You can add spike filters in a sensor’s channel settings. Each filter is valid for the selected channel only. Spike filters are active immediately after saving. They are applied to historic and live data in both tables and graphs.

• Enter a number into the Spike Filter Max. Value. Values above this value will be disregarded.

• Enter a number into the Spike Filter Min. Value field. Values below this value will be disregarded.

As you’re using negative values, please try to exchange the two numbers.

http://www.paessler.com/manuals/prtg8/sensor_channels_settings