A very simple solution would be to:
1. set your "Topics Per Page" to 100.
2. go to the forum you want to delete from.
3. click the "Posted" date column-header to sort them by ascending order.
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4. click the down-arrow for inline moderation (located in the column-header, top-right) and choose "Select All Topics"
5. at the bottom, choose "Delete Topics" and "Go"
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Repeat from #4 for as many posts you want removed.
Doing steps #4 and #5 only ten times, will remove 1,000 topics.
