Depending on what you want there are a couple of options.  Do you want to
completely remove source safe permanently or just make it "feel" that way
when you're working?

1: Completely remove source control permanenently --- lock the SS database,
disconnect from network it's on or in some other way remove the ability for
VS to communicate with SS.  Launch your solution and you will be prompted if
you want to remove from source control, work unconnected etc.  There may be
other ways of doing this, but this is what I can think of right now.
2: Change the source safe options in tools/options | source control and then
environment from the tree --- from there you can choose to check everything
out when you launch the solution, and then check in when closing solution.
You can even choose to keep stuff checked out when checking in, which will
keep the history or all changes, but permanently keep everything checked out
to you.

A final drastic step you can take that I wouldn't recommend is to "manually"
disconnect your solution --- there are many reasons why not to do this, but
the decision is yours.  To do this, find all files with a .scc extension in
your solution folder and rename them.  Then change the read only attribute
of every single file so that it's not read only.  I can't even say for 100%
certain if this would work entirely or as expected.