Restrict Full access to certain User of Database The scenario is the following:


Given SQL Server Instance created and handled not by me
Creation of Database and User on given Instance by me


Question:

To my limited understanding, the sa login will have full access to my Database. Is there a way I can create a user that will be the only user/login that will have full access to my Database, despite the fact I don't own the instance?
If you are trying to prevent the owner of the instance or any Windows or SQL Server administrators from being able to access your database, the answer is no. 

Essentially you should assume that any administrator on the box owns and can gain access to everything on the box, including your database. 

You may be able to protect the actual data in some cases (check out Always Encrypted in SQL Server 2016, for example), but that's about it. The administrator can access everything you don't encrypt (and much of the stuff you do in current versions), drop your database, lock you out, etc.

If you don't trust the service provider that's hosting your database, you should host elsewhere. After all, any host worth their salt should have agreements in place that legally protect you, even though not physically, from internal data breach.