grep -v '^/admin/' removes one prefix. Chaining a dozen grep calls to remove a dozen prefixes rereads the input a dozen times.
One pass with a pattern file
grep -v -f prefixes.txt input.txt reads the patterns from a file and applies them all in a single pass. Add -F if they are literal strings rather than expressions — the combination is both faster and immune to metacharacters in the prefixes.
Alternation as the alternative
grep -vE '^(/admin/|/login/|/tmp/)' does the same in one expression. Readable at three prefixes, unmaintainable at thirty, and the pattern file scales better.
Escaping matters in a pattern file too
Without -F, each line of the file is a regular expression. A prefix containing . or + behaves as a pattern, which usually still matches what you wanted and occasionally matches more. Use -F unless you specifically need patterns.
Try it: Remove Lines Starting With on SeoWolf's Notepad.