with Custom Class

To control what type of objects are placed into a collection, you can create a collection class.
Using a class gives you the ability to create custom replacements for the standard Add, Remove, Count and Item methods.
You can create a class that "wraps up" your collection and then provide the equivalent methods and properties to emulate the buit-in collection object.

Disadvantages

You will loose the default Item method, meaning that you must use the full syntax.

Dim sValue As String 
sValue = myCollection.Item(2)        ' full syntax must be used   
'instead of
sValue = myCollection(2)             ' there is no default method  

You will loose the ability to use a For-Each-Next loop (or enumeration functionality).

Dim lCount As Long 
Dim lCounter As Long 
Dim sValue As String 
lCount = myCollection.Count 
For lCounter = 1 to lCount 
   sValue = myCollection.Item(lCounter) 
Next lCounter 
'instead of
For Each sValue in myCollection      ' for-each cannot be used  
Next sValue 

Class - colStringDataType

Create a new class module called "colStringDataType".

Private colStrings As Collection 

Private Sub Class_Initialize() 
   Set colStrings = New Collection 
End Sub 

Public Function Add(ByVal sText As String) 
   Call colStrigs.Add(Item:=sText, Key:=sText) 
End Function 

Public Sub Remove(ByVal vItem As Variant) 
   colStrings.Remove vItem 
End Sub 

Public Property Get Item(ByVal vItem As Variant) As String 
   Set Item = colStrings(vItem) 
End Property 

Public Property Get Count() As Long 
   Count = colStrings.Count 
End Property 

Here is the code that uses this collection.
This collection can only contain text values and adding something that is not a string will generate an error.

Dim colTextOnly As colStringDataType 
colTextOnly = New colStringDataType 
colTextOnly.Add "mon" 
colTextOnly.Add "tue" 
colTextOnly.Add "wed" 
colTextOnly.Add 200              ' Error