There isn't specifically a way to do this, however if you're comfortable editing files, here are a few steps you can follow:
1. Go to C:\Program Files\Blockland\base\config\client\ and open prefs-trustList.cs using Wordpad.
2. Press Ctrl-F and type in the Name or BL_ID of the person you want to remove. (I'll use MrPickle as this example)
3. You should be presented with a line looking something like this:
$Trust::Line86 = "696\t2\tMrPickle";
4. Now you need to change the 2 (or 1) to a 0, then Save the file.
Notice that this will not take effect until you close and re-open the game.