"I can not get spawneggss to work"
Simply use "monsteregg:<mob id/name>" and install SilkSpawners, like described here: https://www.spigotmc.org/wiki/bossshoppro-itemdata/.
"My players are able to take items out of the GUI"
I'm pretty sure that's not a general bug but either a problem with your setup or an other plugin is conflicting with BSP: There have been a few dupe reports within the past years and all of them have been proven "invalid" so far. Here is a short list of some possible explanations:
- The items are "ghost items" and can not be used by the players. Ghost items are items the player client thinks exist but the server does not. They can be created abusing glitches related to the player client and have nothing to do with the plugin. As the server does not recognize these items they can not be used by the player (except he has explicit permissions, as described in 2. and 3.).
- The player is in creative mode (or switches to creative mode while having ghost items). Players in creative mode in general have the permission to spawn any item (except prevented by other plugins). In that case the server detects ghost items as items "the client wants to spawn" and accepts them.
- The players testing the issue have op permissions which makes them able to spawn any item
- You use a plugin conflicting with BSP: It interferes with the BSP inventory item click event and somehow gives the players the item although BSP cancels the item click event. When players click a BSP gui the item click event is cancelled in any case. The only reason players are able to take items can be other plugins interferring with the event handling of BSP. To find out if that's the case you can remove other plugins step by step and check if the issue remains.