Depending on how you'd like to handle it, you could create a role and grant execute to this role, instead of doing it for each and every user.

select 'GRANT EXECUTE ON ' + name + ' TO XXXXX'
from yourdb..sysobjects
where type = 'P'

and execute it's output to grant the role 'XXXXX' execute on all procedures in 'yourdb' database.

This way you actually make sure that they do NOT have any permissions other than EXECUTE, unless otherwise granted exclusively, which could be tracked and eliminated.

And do not forget to get rid of the GUEST user.