日本語 Japanese Ask Questions Thread 6

If you have a question about the language, ask it and fellow 4channers might see it and answer it for you.

Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/1-40

日語 Japanese Ask Questions Thread2 質問
http://dis.4chan.org/read/lang/1206158123/1-40

日本語 Japanese Ask Questions Thread 3
http://dis.4chan.org/read/lang/1267485093/1-40

日本語 Japanese Ask Questions Thread 4
http://dis.4chan.org/read/lang/1302350850/1-40

日本語 Japanese Ask Questions Thread 5
http://dis.4chan.org/read/lang/1330050873/1-40

So someone tells a joke, the joker gets stopped mid-sentence and the jokeé says(might've heard wrong): あまりにもべかすぎるので、とつさに止めてしまった。 The translation is "I instinctively stopped you since it was so lame".
Since the tranlation seems pretty idiomatic, I wanted to confirm the original meaning.

So, あまりにもべかすぎるので. From the context and translation, I want to say that it's related to the -べく　or -べからず grammar. My guess would be something like "Even if only slighty, you're trying too hard(to make it funny)"

As for とつさに止めてしまった, I'm really just grasping thin air. とつさに just sounds something related to 突然 so I might've guessed "I suddenly(without thinking about my own actions) stopped you.

Bottom line, I need help. I would've been totally stumped had there not been a translation.