The question is not quite clear but I try to answer with my understanding.
So, it looks like you use actor as a job dispatcher which translates job messages to calls for some "job executor system". Each incoming message is translated to some call.
If this call is synchronous (which smells when working with actors of course but just for understanding) then no problem in your case, your actor waits until call is complete, then proceed with next message in its mailbox.
If that call is asynchronous which I guess what you have then all the messages will be handled one by one without waiting for each other.
So you need to throttle the messages handling in order to have at most one message being processed at a time. This can be archived by "pull" pattern which is described here.
You basically allocate one master actor which has a queue with incoming messages (jobs) and one worker actor which asks for job when it is free of jobs. Be careful with the queue in master actor - you probably don't want it to grow too much, think about monitoring and applying back-pressure, which is another big topic covered by akka-stream.