See if the plugin has a 'skip' configuration parameter. Nearly all do. if it does, just add it to a declaration in the child:
<plugin>
<groupId>group</groupId>
<artifactId>artifact</artifactId>
<configuration>
<skip>true</skip>
</configuration>
</plugin>
If not, then use:
<plugin>
<groupId>group</groupId>
<artifactId>artifact</artifactId>
<executions>
<execution>
<id>TheNameOfTheRelevantExecution</id>
<phase>none</phase>
</execution>
</executions>
</plugin>