Is there a way to turn off logging that Hangfire does with serilog?

Define a logger and log provider that does not log anything:

using Hangfire;
using Hangfire.Logging;

public class NoLoggingProvider : ILogProvider {
    public ILog GetLogger(string name) {
        return new NoLoggingLogger();
    }
}

public class NoLoggingLogger : ILog {
    public bool Log(LogLevel logLevel, Func<string> messageFunc, Exception exception = null) {
        return false;
    }
}

and configure Hangfire to use it in your Startup class:

using Hangfire;

public class Startup {
    public void Configuration(IAppBuilder app) {
        GlobalConfiguration.Configuration.UseLogProvider(new NoLoggingProvider());
        // the rest of your configuration...        
    }
}