You need to submit a spend credit request. You can do so here:
http://www.anthemro.com/free/spend.phpIt costs 5 ads to start the process, 2 ads for each card you'd like to have removed, and 2 ads if you'd like to keep the weapon. So, to unslot the DT Garrison, and not keep the weapon, it would cost 9 ads. It would cost 11 ads if you also wanted to keep the weapon.
You only need to keep the weapon on the character you specify in your spend credit request. It doesn't matter if it's equipped, or in his inventory.