Limit default associations

Limit default associations

Hi all,
Newbie alert. Please be gentle :)
We'd like to limit the options of "linked to". 
For example: By default, a call can be linked to either a lead, contact, account or other modules. We'd like to limit these links to specific module. i.e. not allow a call to be linked to an account, only to a contact. 
I hope I explained myself correctly.
Thanks