Sexual Lust is one of the forms of Kāma (or desires). And Kāma is very difficult to control.
However, as per an excerpt from the Devī Bhāgavatapurāṇa, we may deduce that recital of the monosyllabled bījākṣara mantra - "klim or klīṃ ( क्लीं ), may help control and overcome sexual lust.
There's an excerpt in the Devī Bhāgavata Purāṇa, where Indra, out of the fear of losing his Heaven, conspires to disturb the penance of the Nara and Nārāyaṇa Ṛṣis, and employs the god of desires - Kāmamadeva for the task, after several unsuccessful attempts previously.
Book 4, Devī Bhāgavatapurāṇa
व्यास उवाच इति तस्य वचः श्रुत्वा शक्रं प्राह मनोभवः । वासवाद्य
करिष्यामि कार्यं ते मनसेप्सितम् ॥ ४६ ॥
- Vyāsa said :-- Kāmadeva, on hearing the king of the Devas, addressed him thus :-- O Indra! Today I will fulfill all your desires.
यदि विष्णुं महेशं वा ब्रह्माणं वा दिवाकरम् । ध्यायन्तौ तौ तदास्माकं
भवितारौ वशौ मुनी ॥ ४७ ॥
- But there is one word. If these two ascetics be meditating Viṣṇu, Śiva or Brahmā or the Sun, then I will be able to bring them under my
देवीभक्तं वशीकर्तुं नाहं शक्तः कथञ्चन । कामराज महाबीजं चिन्तयन्तं
मनस्यलम् ॥ ४८ ॥
- And if they be meditating on the Great Seed Mantra, the root of all Māyā, and the great Kāmavījam (klim/klīṃ - क्लीं), the king of the Kāma, I will never
be able to subdue such a devotee of the Highest Devī.
तां देवीं चेन्महाशक्तिं संश्रितौ भक्तिभावतः । न तदा मम बाणानां गोचरौ
तापसौ किल ॥ ४९ ॥
- If these two ascetics have devotedly taken refuge of the Great Power Mahā Devī, then they will not come under the sight of my arrows.
English translation by Swami Vijñanananda
From this excerpt, we may conclude that the practitioner of the Kāmavījam which is "klim or klīṃ ( क्लीं ), the bīja-mantra of the Great Mahā Devī, can control lustful desires and thoughts.
Of course, one must be initiated into these mantras, by a qualified preceptor, for them to be effective.
But nonetheless, as per the above excerpt from the Devī Bhāgavatapurāṇa, the monosyllabled bījākṣara mantra -
"klim or klīṃ ( क्लीं ) may be said to be effective in reducing sexual lust or any forms of Kāma (or desires) in general.