As far as I know, the "!important" modifier must be specified per property. So, if you want both margins to be high priority, you should use:
Code:
p { margin-top: 0.3em !important ; margin-bottom: 0.3em !important }
or, if you want no side margins, this is shorter:
Code:
p { margin: 0.3em 0 !important }
(When you use "margin" with two values, the first value applies to top and bottom margins, the second value applies to left and right margins. When a value is 0 you don't have to specify the units.)