Whacking a Weevil with a stick isn’t a sensible thing to do. It doesn’t harm the Weevil, just pisses it off so it turns on Ianto. On the bright side, it stops trying to rip Harkness’s throat out, which was why he intervened in the first place; Ianto needs the leader of Torchwood Three alive.
On the downside, Ianto’s now wrestling a Weevil, and losing ground until Harkness manages to subdue it.
“And you are?”
“Jones, Ianto Jones.” Ianto mentally kicks himself. ‘Oh, well done, you moron! Great first impression! Now he’ll think you’re some sort of James Bond wannabe.’