[CentOS] BIND (named) as secondary and .jnl files

Hello,

I have two running BINDs in my LAN, one on my router box and one as VM;
both are caching DNS servers, and a few zones are on both, on the box as 
master and on the VM as slave,
but how can I cleanup/flush the growing .jnl files;

rndc freeze
rndc thaw

doesn't work as these are not master ...

Thanks,
Walter