I'm not sure if it's doable from forum manager, but it's certainly doable. The templates you need to modify are the:
forumhome_forumbit_level1_post
forumhome_forumbit_level2_post
Find in the templates where it is posting the last post info. Should look something like this:
Code:
<td class="alt2" nowrap="nowrap">$forum[lastpostinfo]</td>
replace it with:
Code:
<td class="alt2" nowrap="nowrap"><if condition="$forum[forumid] == 17">-<else />$forum[lastpostinfo]</if></td>
This does it for forum #17, change it to whichever forum or forums you'd like to blank.