This is a simple one. On the website, go to their profile and click on “Block this member” in the left column.
On the mobile app, go to their profile and click in the upper right corner, then click “Block” in the pop-up.
Keep in mind that once you block someone, there’s a 72-hour waiting period before you can unblock them (this is to prevent abuse of the block feature).
Hope this helps,
The Grokio Team