Not sure which your using or your provider, but with AT&T, upper right corner 3 dots, tap on that and see Search; Saved Box; Spam Manager; Settings. In a text message, the three dots and it has Register Spam number near the bottom. This is with the stock messaging app.
I have Sprint and do not see that option when I click the three dots on the top right. This is going to be terrible if I do not have it just because of the provider.
I have Verizon & I have not found a way to block numbers on my phone either; but I know that you can from your Verizon Account. Sprint may be the same. On my Verizon Note 3, I could block numbers on the phone. You can mark as "spam" on the G4 with both the stock Email & Gmail apps.
I have Sprint and do not see that option when I click the three dots on the top right. This is going to be terrible if I do not have it just because of the provider.