@kgroneman - Unfortunately there is no setting available to limit a label selection on the community admin. There is an idea for this as well https://community.lithium.com/t5/Product-Ideas/Limit-number-of-Labels-users-can-select/idi-p/102367
However, this can be achieved using some customization. You required JS script to limit a single label. Also, just do you have predefined labels for idea exchange?
Give kudos if you find my posts helpful or mark solution if it answers your query.
Tariq
