RE: How to hide displaying custom entity from lookup field
Hi,
You have to export data that you currently have i this table, then redesign the entity and reimport the data. Remember that if this entity has relationships with other entities, you will also loose this information from related entities, so you have to take a full backup of your database and then refill the data after redesign. This is very difficult process, if you don't have much experience with CRM I would not recommend this.
Easier version would be to create completely new entity without Activities. Not you have both entities in your system. Migrate all data from old entity to new one (using CRM SDK). Now you can make sure that you did not missed any data. When you are sure new entity has all the date, you can remove the old entity. Of course in such scenarion the new entity will have different name.