Might be time to look for another hosting provider.
A defunct process is a process that has exited but still has an entry in the kernels process table. In most unices it has no size other than the sizeof() the entry itself. There is one unix variant that does store some resident information about the process other than its table entry but I cant remember which one offhand. This should not kill the server unless there are so many of them that they fill the process table in which case all fork()s will return errors until there is space for thier new processes in the table.
Are these processes exiting after a while or are they staying around? These processes are normal in that all processes besides init become defunct when they exit() and are cleaned from the process table by init or the kernel. Logic would then suggest that the cleaner-upper (very technical term there) isn't doing its job. =)
So if I were you, I would look for another hosting provider.