In the case of Towers, you can do that by setting the EnableTowerSelectionAtLevelStart? variable in **BP_GameMode_TowerDefense **to False. If you want the same for Global Abilities as well, the following modifications should do the trick.
Also, make sure to update the **BaseTowerModels **array in **BP_TowerManager **and **AvailableGlobalAbilities **array in **BP_GlobalAbilitiesManager **when you add or remove new Towers/Abilities.