As i understand it, summon requires the presence of a token in the inventory of the summoned player. Unwilling reception of items has been an issue that's surfaced in darke a number of times, and there seems to be a simple solution. In a manner similar to one player allowing another player to follow, there could be a command implemented where one player would have to allow another player to give them items. Are you going to accept something from someone if you don't want it in your possession? Probably not. Simple syntax:
trade allow <name>
Toggles the ability of <name>d player to give items to the one entering the command.
Of course, the stealth-requiring act of slipping something small into another person's inventory could be, and probably should be, a guild ability, if it is determined that the ability to do so will serve a useful purpose within the game.
Shadowraith has kindly provided a tool for use here. We have the 'receive' command, which provides us with a solution that may please none, and should then stand as a viable option. If the receive command is toggled to disallow others from giving items to you, it stands to reason that this can be portrayed as an individual being very vigilant about their own inventory. Perhaps we see the common street urchin with their tattered rags clutched tightly to themselves as a prime example of one who would fail to notice an invisible mage slipping them a small item, but would at some point feel a difference in their inventory and immediately relieve themselves of unwanted items.