error when redeploying to local jetty8

hi,
I have problem when I redeploy war to local jetty (when I select 'update resources on frame deactivation' or I choose artifacts to redeploy).

it throws

Error copying 'myproj\src\main\webapp\less\defaults.less' to 'myproj\target\myproj.war
\less\defaults.less'.: myproj\target\myproj.war\less\defaults.less 
(The requested operation cannot be performed on a file with a user-mapped section open)

from this post http://stackoverflow.com/questions/4988019/jetty-maven-plugin-is-ignoring-custom-webdefault-xml I ve found out that I should configure jetty to stop lock resources but I cannot force jetty to do so.

I tried to override configuration in web.xml or in webdefault.xml

<init-param>
  <param-name>useFileMappedBuffer</param-name>
  <param-value>false</param-value>
</init-param>

but it has no effect. my jetty server is configured using jetty-jmx.xml file inside intellij and it throws error when I try to to change it to something different. I tried to set 

useFileMappedBuffer 

jetty-jmx.xml but I cant find the proper place.

I would appreciate any feedback on this,

thanks