See the Pelican / 101 Projects write up:
Pelican Technical Article: Boxster Rear Spoiler Installation - 986 / 987
"For pre-2001 cars, you can eliminate the spoiler warning lamp by removing the bulb in the gauge cluster (see Pelican Technical Article: 996 Gauge Cluster Upgrade). For 2001-04 cars which no longer use light bulbs in the cluster, you can trick the car into thinking the spoiler is still attached. Pull the two brown relays and disconnect the connection to the spoiler (red arrow), located under the trunk carpet. Then remove pin 1 (blue/green wire) from the connector, and wrap it in electrical tape. Then swap pin 2 (gray/brown wire) and pin 4 (gray/green wire) and replug it in. This should stop the warning lamp from illuminating in the instrument cluster."