Understanding how to use 以後
&quot;以后再也不来这里吃饭了，又贵又难吃&quot; is grammatically correct because it does modify an event, and the event is 'from now on not come here for dinner'.
This sentence implies &quot;從今(from now on)以后再也不来这里吃饭了，又贵又难吃&quot;
'從今'(from now on) is omitted because we know '以后'(hereafter) is referring to 'from now, hereafter'
Just like if you wrote &quot;(昨天在这里吃了一餐)以后，再也不會来这里吃饭了，又贵又难吃&quot; 以后 modifies the event &quot;昨天在这里吃了一餐&quot;.

2) Would removing the commas for the second example make it grammatically correct

我六点下班以后，我去跟朋友吃饭 is technically correct, but it should be a single sentence: &quot;我六点下班(之后)去跟朋友吃饭&quot;
Use 之后 instead of 以后 because 以后 is usually followed by a unique event
Example to illustrate my point:

清朝灭亡(以后), 中国再没有皇帝了 (correct)
清朝灭亡(之后), 中国再没有皇帝了 (correct)

'The demise of the Qing Dynasty; no more emperor' is a unique event, you cannot have this event again

雨停(以后)才能出外 (incorrect)
雨停(之后)才能出外 (correct)

'after the rain stop; going out' is not a unique event, it will happen again and again
Best way is to simply write: &quot;我六点下班(后)去跟朋友吃饭&quot;