This problem has been there for some time and several new versions. I am nearly to the point of disabling Boinc top get rid of the annoying bother.

System:

Mid 2010 Mac Pro tower case
OS version 10.12.6
Boinc version 7.14.2
No GPU

Log:

Fri Dec 28 10:36:57 2018 | | cc_config.xml not found - using defaults
Fri Dec 28 10:36:59 2018 | | Starting BOINC client version 7.14.2 for x86_64-apple-darwin
Fri Dec 28 10:36:59 2018 | | log flags: file_xfer, sched_ops, task
Fri Dec 28 10:36:59 2018 | | Libraries: libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.8 c-ares/1.13.0
Fri Dec 28 10:36:59 2018 | | Data directory: /Library/Application Support/BOINC Data
Fri Dec 28 10:37:00 2018 | | OpenCL: AMD/ATI GPU 0: ATI Radeon HD 5770 (driver version 1.2 (Nov 4 2018 20:33:15), device version OpenCL 1.2, 1024MB, 1024MB available, 680 GFLOPS peak)
Fri Dec 28 10:37:00 2018 | | OpenCL CPU: Intel(R) Xeon(R) CPU W3565 @ 3.20GHz (OpenCL driver vendor: Apple, driver version 1.1, device version OpenCL 1.2)
Fri Dec 28 10:37:00 2018 | | Host name: RobinsOwn.local
Fri Dec 28 10:37:00 2018 | | Processor: 8 GenuineIntel Intel(R) Xeon(R) CPU W3565 @ 3.20GHz [x86 Family 6 Model 26 Stepping 5]
Fri Dec 28 10:37:00 2018 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clfsh ds acpi mmx fxsr sse sse2 ss htt tm pbe pni dtes64 mon dscpl vmx est tm2 ssse3 cx16 tpr pdcm sse4_1 sse4_2 popcnt
Fri Dec 28 10:37:00 2018 | | OS: Mac OS X 10.12.6 (Darwin 16.7.0)
Fri Dec 28 10:37:00 2018 | | Memory: 20.00 GB physical, 614.77 GB virtual
Fri Dec 28 10:37:00 2018 | | Disk: 931.19 GB total, 614.53 GB free
Fri Dec 28 10:37:00 2018 | | Local time is UTC -8 hours
Fri Dec 28 10:37:00 2018 | SETI@home | URL http://setiathome.berkeley.edu/; Computer ID 5588240; resource share 100
Fri Dec 28 10:37:00 2018 | SETI@home | General prefs: from SETI@home (last modified 02-Dec-2007 16:47:52)
Fri Dec 28 10:37:00 2018 | SETI@home | Host location: none
Fri Dec 28 10:37:00 2018 | SETI@home | General prefs: using your defaults
Fri Dec 28 10:37:00 2018 | | Reading preferences override file
Fri Dec 28 10:37:00 2018 | | Preferences:
Fri Dec 28 10:37:00 2018 | | max memory usage when active: 10240.00 MB
Fri Dec 28 10:37:00 2018 | | max memory usage when idle: 18432.00 MB
Fri Dec 28 10:37:00 2018 | | max disk usage: 40.00 GB
Fri Dec 28 10:37:00 2018 | | don't compute while active
Fri Dec 28 10:37:00 2018 | | don't use GPU while active
Fri Dec 28 10:37:00 2018 | | suspend work if non-BOINC CPU load exceeds 25%
Fri Dec 28 10:37:00 2018 | | max download rate: 51200 bytes/sec
Fri Dec 28 10:37:00 2018 | | max upload rate: 51200 bytes/sec
Fri Dec 28 10:37:00 2018 | | (to change preferences, visit a project web site or select Preferences in the Manager)
Fri Dec 28 10:37:00 2018 | | Setting up project and slot directories
Fri Dec 28 10:37:00 2018 | | Checking active tasks
Fri Dec 28 10:37:01 2018 | | Setting up GUI RPC socket
Fri Dec 28 10:37:01 2018 | | Checking presence of 144 project files
Fri Dec 28 10:37:01 2018 | | Suspending computation - computer is in use
Fri Dec 28 10:37:01 2018 | | Suspending network activity - computer is in use

Don't know if this is just a Mac problem in which case, I am completely ignorant. Do any messages come up when you try and shut down without doing a force quit first?

No clue, haven't seen that on my Macs ever.

Which part(s) of BOINC are needing the force quit? "boinc" "BOINCManager" or the science applications (project specific)

(Sudden back of brain question)
Question, do you have multiple users on your Mac?

Have you tried logging out and then shutdown?