You can remove OTR plugin from Spark installation folder
C:\Program Files (x86)\Spark\plugins\otrplug.jar
C:\Users\wroot\AppData\Roaming\Spark\plugins\otrplug\
C:\Users\wroot\AppData\Roaming\Spark\plugins\otrplug.jar
Also, per this document instructions you can add OTR to plugins blacklist in default.properties file and then distribute customized spark.jar to clients.