jansohn's solution works for me.
I pulled the mmssms.db file from a previous NANDROID backup of my phone and was able to restore all the old SMS messages from it by doing those commands in an adb shell.
It didn't work until I used:
adb shell chcon u:object_r:radio_data_file:s0 /data/user_de/0/com.android.providers.telephony/databases/mmssms.db
I think it might work to misspell it and then not press spacebar. When the suggestion pops up (on my Gboard, there are always corrections at the top of the keyboard) you can see the word you do not want it to change to if you were to press spacebar. You then long-press that suggestion and then it gives you an option to remove that suggestion. I just did it, ...