Fulltext search limiting users possibilities
Here are three suggestions.

Suggestion 1. Use plainto_tsquery when you convert the user provided search string to PostgreSQL's tsquery, see the PostgreSQL docs. — I think that plainto_tsquery discards any special magical characters + AND, OR etcetera. (But I'm not completely sure. You'd better verify this so you don't do anything dangerous)  Oops I didn't notice that this was about SQL Server not PostgreSQL.

Suggestion 2. Build a HTML form for advanced search queries, with fields like:

Include all these words: ...
Include any of these words: ...
Exclude all these ...

And so on. Next, construct the otherwise dangerous full text search expression yourself in SQL Server syntax.

Suggestion 3. You could provide a single search text field, and let e.g. "-someword" mean that "someword" should be excluded, and "+anotherword" mean that "anotherword" must be included. Then, you build your own parser for this search field, and convert the resulting "abstract search tree" to something SQL Server understands, and give it to SQL Server.