Open your phone app, switch to the call log tab. Look for the entry you want to block and tap that. You should see a button/link labeled "more" or "details", tap that. On the next page (which shows details on the call), tap the "overflow menu" on the top-right of the screen (the "3 stapled dots"). Select "Add to block list". Done.
Note: I've verified that on my device running Lollipop (Android 5.1). Still, this feature might not be available on all devices. According to an article on DigitalTrends:
There is no universal path for blocking contacts on Android smartphones, and it’s a feature the vanilla versions of Android Lollipop and Marshmallow oddly omit. But some devices allow users to enable a per-number blocking mode through the phone’s settings.
So if that feature does not exist on your device, first cross-check with the linked article for possible other locations. If it really cannot be found, you'll have no choice but to revert to a third-party-app for this feature.
Btw, for LG (as you mention your device is an LG device) the linked article writes:
LG’s made it relatively simple to build an auto-reject list on its handsets. To start, open the Phone app. Tap the 3-dot menu in the top right corner, and from the dropdown menu that appears select Call Settings > Call reject > Reject calls from. Select the Plus icon, and then add the number you’d like to block from the call log or your contacts. You can also find it by going to Settings > Wireless and networks > Call > Call reject > Reject calls from.